diff --git a/src/module/errors.js b/src/module/errors.js
new file mode 100644
index 0000000..3db0685
--- /dev/null
+++ b/src/module/errors.js
@@ -0,0 +1,19 @@
+/**
+ * Placeholder class for an error that is thrown when a ship is placed
+ * in an invalid position.
+ * @extends Error
+ */
+export class InvalidShipPlacementError extends Error {}
+
+/**
+ * Error thrown when the program tries to advance the state, but it is
+ * invalid.
+ * @extends Error
+ */
+export class InvalidAdvanceStateError extends Error {}
+
+/**
+ * Error thrown when a missile is fired at an invalid cell.
+ * @extends Error
+ */
+export class InvalidMissileFireAttemptError extends Error {}
diff --git a/src/module/util.js b/src/module/util.js
index 487e788..cf2d52e 100644
--- a/src/module/util.js
+++ b/src/module/util.js
@@ -22,6 +22,31 @@ export const GridCellState = {
     Missed: 'missed',
 }
 
+/**
+ * Returns true if the given grid cell state represents a ship in some way.
+ * @param {GridCellState} grid_cell_state
+ * @return {boolean}
+ */
+export function isShipCell(grid_cell_state) {
+    return [
+        GridCellState.Ship,
+        GridCellState.Damaged,
+        GridCellState.Sunk,
+    ].includes(grid_cell_state)
+}
+
+/**
+ * Returns true if the given grid cell state can be fired upon.
+ * @param {GridCellState} grid_cell_state
+ * @return {boolean}
+ */
+export function isValidTargetCell(grid_cell_state) {
+    return [
+        GridCellState.Ship,
+        GridCellState.Available,
+    ].includes(grid_cell_state)
+}
+
 /**
  * Enum of all possible players.
  * @type {object}
@@ -52,6 +77,22 @@ export const GameState = {
     PlayerVictory: 'player_victory',
 }
 
+/**
+ * The various supported ship types, by dimension.
+ * @type {object}
+ */
+export const ShipType = {
+    x1: '1x1',
+    x2: '1x2',
+    x3: '1x3',
+    x4: '1x4',
+    x5: '1x5',
+}
+
+export function isShipType(type) {
+    return ['1x1', '1x2', '1x3', '1x4', '1x5'].includes(type)
+}
+
 /**
  * Makes a deep copy of the value passed in.
  * @param {*} obj
diff --git a/src/services/GameState.service.js b/src/services/GameState.service.js
index 5195d67..572a635 100644
--- a/src/services/GameState.service.js
+++ b/src/services/GameState.service.js
@@ -1,4 +1,5 @@
-import { Player, GridCellState, GameState, clone } from '../module/util.js'
+import { Player, GridCellState, GameState, clone, ShipType, isShipType, isShipCell, isValidTargetCell } from '../module/util.js'
+import { InvalidShipPlacementError, InvalidAdvanceStateError, InvalidMissileFireAttemptError } from '../module/errors.js'
 
 /**
  * Singleton service for managing the state of the game.
@@ -11,6 +12,13 @@ export class GameStateService {
      */
     player_x_game_board = {}
 
+    /**
+     * A mapping of player => ship definitions.
+     * @private
+     * @type {object}
+     */
+    player_x_ships = {}
+
     /**
      * An array of all players. This is mostly for internal purposes.
      * @private
@@ -32,6 +40,34 @@ export class GameStateService {
      */
     n_cols = 9
 
+    /**
+     * Number boats placed on the board.
+     * @private
+     * @type {number}
+     */
+    n_boats = 1
+
+    /**
+     * gets the number of boats placed on the board
+     * @private
+     * @return {number}
+     */
+    get_n_boats(){
+        return (this.n_boats);
+    }
+
+    /**
+     * sets the number of boats to a valid number 
+     * @private
+     * @return none 
+     */
+    set_n_boats(number){
+        if(number >= 1 && number <= 5 )
+        {
+            this.n_boats = number; 
+        }
+    }
+
     /**
      * The current state of the game.
      * @private
@@ -60,6 +96,7 @@ export class GameStateService {
         // Generate empty boards for each player
         for ( const player of this.players ) {
             this.player_x_game_board[player] = this._build_empty_board()
+            this.player_x_ships[player] = []
         }
     }
 
@@ -124,6 +161,326 @@ export class GameStateService {
         })
     }
 
+    /**
+     * get the "score" (the number of hits) that the 
+     * current player has (counting sunk ships)
+     * @return {number}
+     * @private 
+     */
+    get_player_score () {
+        let i = 1;
+        let j = 1;
+        let score = 0;
+        for(i; i<=9; i++)
+        {
+            for(j; j<=9; j++)
+            {
+                let cell = this.player_x_game_board[this.current_opponent][i][j];
+                if(cell.render === GridCellState.Damaged || cell.render === GridCellState.Sunk )
+                {
+                    score++;
+                }
+            }
+        }
+        return(score);
+    }
+
+    /**
+     * gets the number of the boats (sunken, damaged or not) that the opponent has
+     * used to help keep get progress method looking clean
+     * @return {number}
+     * @private 
+     */
+    get_boat_count(){
+        let i = 1;
+        let j = 1;
+        let boat_count = 0;
+        for(i; i<=9; i++)
+        {
+            for(j; j<=9; j++)
+            {
+                let cell = this.player_x_game_board[this.current_opponent][i][j];
+                if(cell.render === GridCellState.Damaged || cell.render === GridCellState.Sunk || cell.render === GridCellState.Ship )
+                {
+                    boat_count++;
+                }
+            }
+        }
+        return(boat_count);
+    }
+
+    /**
+     * gets the progress (hits/total boats) that the player has
+     * @return {number}
+     * @private 
+     */
+    get_progress(){
+        return(this.get_player_score() / this.get_progress() )
+    }
+
+
+    /**
+     * responsible for advancing the game state 
+     * will be consisting of 
+     * @return 
+     * @private 
+     */
+    advance_game_state() {
+        /** functions to be made that validate:
+         * 1) number of ships
+         * 2) player one placement
+         * 3) player two placement
+         * 4) player one turn
+         * 5) advance to player 2
+         * 6) player 2 turn
+         * 7) advance to player one
+         * 8) player win
+         *
+         */
+        //1
+        if (this.current_state === GameState.ChoosingNumberOfShips) {
+            if (this.n_boats >= 1 && this.n_boats <= 5) {
+                this.current_state = GameState.PlayerSetup;
+                this.current_player = Player.One;
+                this.current_opponent = Player.Two;
+            } else {
+                throw new InvalidAdvanceStateError("Invalid Number of Boats");
+            }
+
+        }
+        if (this.current_state === GameState.PlayerSetup) {
+            if (this.current_player === Player.One) {
+                //wait
+                // because the place_ship handles all the validation
+                // all you need to do is make sure they have placed all the appropriate ships
+                // e.g. if ( this.get_ship_entities(this.current_player).length === this.n_boats ) { ... }
+            }
+            if (this.current_player === Player.Two) {
+                //wait for now
+            }
+        }
+    }
+
+    /**
+     * Attempt to fire a missile at the current opponent at the given coordinates.
+     * The coordinates should be an array of [row_index, column_index] where the missile should fire.
+     * Returns true if the missile hit an undamaged cell of a ship.
+     *
+     * @example
+     * If I want to fire a missile at row 5 column 7, then:
+     * game_service.attempt_missile_fire([5, 7])
+     *
+     * @param {[number, number]} coords
+     * @return {boolean}
+     */
+    attempt_missile_fire([target_row_i, target_col_i]) {
+        const target_cell = this._get_cell_state(this.current_opponent, target_row_i, target_col_i)
+        if ( !isValidTargetCell(target_cell.render) )
+            throw new InvalidMissileFireAttemptError('Cannot fire on cell with state: ' + target_cell.render)
+
+        if ( target_cell.render === GridCellState.Ship ) {
+            // We hit an un-hit ship cell!
+            this._set_cell_state(this.current_opponent, target_row_i, target_col_i, GridCellState.Damaged)
+
+            // set ships to sunk where appropriate
+            this._sink_damaged_ships(this.current_opponent)
+            return true
+        } else if ( target_cell.render === GridCellState.Available ) {
+            // We missed...
+            this._set_cell_state(this.current_opponent, target_row_i, target_col_i, GridCellState.Missed)
+        }
+
+        return false
+    }
+
+    /**
+     * Checks the player's ships. If any are fully damaged, it flags that ship's cells
+     * as "sunk" rather than damaged.
+     * @param {Player} player
+     * @private
+     */
+    _sink_damaged_ships(player) {
+        this.get_ship_entities(player).some(ship => {
+            const covered_cells = this.get_covered_cells(ship.coords_one, ship.coords_two)
+            const all_damaged = covered_cells.every(([cell_row, cell_col]) => {
+                return this._get_cell_state(player, cell_row, cell_col).render === GridCellState.Damaged
+            })
+
+            if ( all_damaged ) {
+                // The entire boat was damaged, so sink it
+                covered_cells.some(([cell_row, cell_col]) => {
+                    this._set_cell_state(player, cell_row, cell_col, GridCellState.Sunk)
+                })
+            }
+        })
+    }
+
+    /**
+     * Attempt to place a ship of the given type at the given coordinates.
+     * Throws an InvalidShipPlacementError if the coordinates are invalid.
+     * Coordinates should be [row_index, column_index] of either end of the ship.
+     *
+     * @example
+     * If I am placing a 1x3 ship and I want it to be in row 3 column 2 horizontal
+     * to row 3 column 4, then I would call:
+     * game_service.place_ship(ShipType.x3, [3,2], [3,4])
+     *
+     * @param {ShipType} ship_type
+     * @param {[number, number]} coords_one
+     * @param {[number, number]} coords_two
+     */
+    place_ship(ship_type, coords_one, coords_two) {
+        // make sure the coordinates are valid for the given ship type
+        this.validate_coordinates(ship_type, coords_one, coords_two)
+
+        // get the ships for the current player
+        const player_ships = this.get_ship_entities(this.current_player)
+
+        // make sure they don't already have this ship type
+        const have_ship_type = player_ships.some(ship => ship.ship_type === ship_type)
+        if ( have_ship_type )
+            throw new InvalidShipPlacementError('A ship with this type has already been placed.')
+
+        // make sure they don't already have too many
+        if ( player_ships.length >= this.n_boats )
+            throw new InvalidShipPlacementError('This player cannot place any more ships.')
+
+        // place the ship
+        this.player_x_ships[this.current_player].push({ ship_type, coords_one, coords_two })
+
+        // mark the cells as having a ship in them
+        this.get_covered_cells(coords_one, coords_two).some(([row_i, col_i]) => {
+            this._set_cell_state(this.current_player, row_i, col_i, GridCellState.Ship)
+        })
+    }
+
+    /**
+     * Get an array of cell coordinates that are covered by a ship that spans
+     * the given coordinates.
+     *
+     * @example
+     * If a ship goes from row 1 column 1 to row 4 column 1, then I can get
+     * the coordinates of all cells covered by that ship using:
+     * game_service.get_covered_cells([1,1], [4,1])
+     * Which would return [[1,1], [2,1], [3,1], [4,1]].
+     *
+     * @param {[number, number]} coords_one
+     * @param {[number, number]} coords_two
+     * @return {[number, number][]}
+     */
+    get_covered_cells(coords_one, coords_two) {
+        const [row_one, col_one] = coords_one
+        const [row_two, col_two] = coords_two
+        const [left_col, right_col] = [Math.min(col_one, col_two), Math.max(col_one, col_two)]
+        const [top_row, bottom_row] = [Math.min(row_one, row_two), Math.max(row_one, row_two)]
+        const is_horizontal = top_row === bottom_row
+
+        if ( is_horizontal ) {
+            return Array((right_col - left_col) + 1).fill('').map((_, i) => {
+                return [top_row, i + left_col]
+            })
+        } else {
+            return Array((bottom_row - top_row) + 1).fill('').map((_, i) => {
+                return [i + top_row, left_col]
+            })
+        }
+    }
+
+    /**
+     * Validate the given coordinates for the given ship type.
+     * Throws an InvalidShipPlacementError if the coordinates are invalid.
+     * Coordinates should be [row_index, column_index] of either end of the ship.
+     *
+     * @example
+     * If I am placing a 1x3 ship and I want it to be in row 3 column 2 horizontal
+     * to row 3 column 4, then I would call:
+     * game_service.validate_coordinates(ShipType.x3, [3,2], [3,4])
+     *
+     * @param {ShipType} ship_type
+     * @param {[number, number]} coords_one
+     * @param {[number, number]} coords_two
+     */
+    validate_coordinates(ship_type, coords_one, coords_two) {
+        if ( !isShipType(ship_type) ) throw new InvalidShipPlacementError('Invalid ship type: '+ship_type)
+
+        const ship_length = this.get_ship_length(ship_type)
+        const [row_one, col_one] = coords_one
+        const [row_two, col_two] = coords_two
+        const [left_col, right_col] = [Math.min(col_one, col_two), Math.max(col_one, col_two)]
+        const [top_row, bottom_row] = [Math.min(row_one, row_two), Math.max(row_one, row_two)]
+
+        const is_horizontal = top_row === bottom_row
+        const ship_cells = this.get_ship_cells(this.current_player)
+        const placement_cells = []
+
+        if ( is_horizontal ) {
+            // Make sure the input length matches the given ship type
+            if ( (right_col - left_col) !== (ship_length - 1) )
+                throw new InvalidShipPlacementError('Invalid coordinates: ship length is invalid')
+
+            Array(ship_length).fill('').map((_, i) => {
+                placement_cells.push([top_row, i + left_col])
+            })
+        } else {
+            // Make sure the input length matches the given ship type
+            if ( (bottom_row - top_row) !== (ship_length - 1) )
+                throw new InvalidShipPlacementError('Invalid coordinates: ship length is invalid')
+
+            Array(ship_length).fill('').map((_, i) => {
+                placement_cells.push([i + top_row, left_col])
+            })
+        }
+
+        // Make sure none of the placement cells overlap with existing ships
+        const has_overlap = ship_cells.some(([ship_row, ship_col]) => {
+            return placement_cells.some(([placement_row, placement_col]) => {
+                return ship_row === placement_row && ship_col === placement_col
+            })
+        })
+
+        if ( has_overlap )
+            throw new InvalidShipPlacementError('Invalid coordinates: ship overlaps with others')
+    }
+
+    /**
+     * Get the number of cells the given ship type should occupy.
+     * @param {ShipType} ship_type
+     * @return {number}
+     */
+    get_ship_length(ship_type) {
+        if ( ship_type === ShipType.x1 ) return 1
+        if ( ship_type === ShipType.x2 ) return 2
+        if ( ship_type === ShipType.x3 ) return 3
+        if ( ship_type === ShipType.x4 ) return 4
+        if ( ship_type === ShipType.x5 ) return 5
+    }
+
+    /**
+     * Get the coordinates of all cells that have ships in them, for the given player.
+     * @param {Player} player
+     * @return {[number, number]}
+     */
+    get_ship_cells(player) {
+        const cells = []
+        this.player_x_game_board[player].some((row, row_i) => {
+            row.some((col, col_i) => {
+                if ( isShipCell(col.render) ) {
+                    cells.push([row_i, col_i])
+                }
+            })
+        })
+        return cells
+    }
+
+    /**
+     * Get an array of ship entities for the given player.
+     * @param {Player} player
+     * @return {object[]}
+     */
+    get_ship_entities(player) {
+        return clone(this.player_x_ships[player])
+    }
+
     /**
      * Build an empty structure of grid cells.
      * @return {object[][]}
@@ -138,6 +495,31 @@ export class GameStateService {
             })
         })
     }
+
+    /**
+     * Set the state of the cell at the given coordinates on the player's board
+     * to the specified state.
+     * @param {Player} player
+     * @param {number} row_i
+     * @param {number} col_i
+     * @param {GridCellState} state
+     * @private
+     */
+    _set_cell_state(player, row_i, col_i, state) {
+        this.player_x_game_board[player][row_i][col_i].render = state
+    }
+
+    /**
+     * Get the state of the cell at the given coordinates on the player's board.
+     * @param {Player} player
+     * @param {number} row_i
+     * @param {number} col_i
+     * @return {object}
+     * @private
+     */
+    _get_cell_state(player, row_i, col_i) {
+        return this.player_x_game_board[player][row_i][col_i]
+    }
 }
 
 // Export a single instance, so it can be shared by all files