In Yak, the village elder has given you (and the other players) the task of constructing a great stone tower to guide the merchants and their yaks in the Himalayas. Every turn, a yak pulls its cart into your village. Will you get stones for your tower, or food for your reserves? Or will you need to visit the market to locate what you need?

Over the course of the game, every player builds their own stone tower by acquiring stones from visiting merchants. You begin the game with one good of each type (meat, milk, bread) and three cards in hand: build, restock, and market. Three or four carts being pulled by yaks begin on the game board, with each player having one cart in front of them and each cart containing three stones and some food. Every cart has a restriction on it, e.g., "no bread" because its yak is gluten intolerant.

On a turn, all players pick a card in hand simultaneously and place it face-down. Beginning with whoever holds the baby yak token, players then take their turns in order by revealing their card. If you picked:

• Build: Spend 1-5 food tokens, putting them in the cart in front of you, to buy up 1-3 stones from that cart. You must follow the restriction for that cart. (No meat for the vegetarian yak, please!) If you get crystal stones, you must pay one extra food for each such stone. Put the stones in your tower, with all stones after your first needing to touch. Your tower will be at most five stones wide.
• Restock: Get all the food of one type from the cart in front of you and add it to your reserves up to your limit of eight items of food. Then take a stone from the quarry bag and place it in the cart. (A cart can hold at most four stones.)
• Market: Get two food items of your choice from the market, which starts with two foods of each type and which is refilled only when a cart exceeds its nine-food limit during a build action. Then take three stones from the bag, place one in a cart of your choice, then return the other two to the bag.

Stones come in eight different colors, with eight of each in the bag. Eight crystal stones are also in the bag, as well as five fog stones. Whenever fog stones are drawn, all players will reverse the direction of all the carts, then the active player sets the fog aside and draws replacement stones, with carts reversing again should fog be drawn anew. Once the right number of non-fog stones have been drawn, put the fog stones on the mountain track. This track will be filled twice during the game, after which only a single fog will be left in the bag.

To end a round, return your selected card to your hand, move the yak carts one space in the direction they face, then move the baby yak clockwise. Continue playing rounds until someone finishes the fourth level of their tower. Finish that round, play one additional round, then score the towers. A crystal stone can be a color of your choice, but every crystal in your tower must be a different color. You get points (1) for each group of two or more stones (e.g., a group of six is worth 16 points), (2) for the number of groups of at least two or more stones, (3) for individual stones (only 1 point each), (4) for triggering the end of the game, and (5) for having the most remaining food.

For an additional challenge, you can play Yak with the "master builder" variant. Show three of the eight objective cards at the start of play. By having the most non-crystal stones of a color or putting them in certain locations in your tower, you score additional points.