The deck consists of cards of 4 colors: red, green, blue, and yellow. The ranks in each color are 0-9. There are 3 "action" cards in each color, labeled "skip", "draw two", and "reverse". There are also special black action cards, "wild" and "wild draw four". There are two copies of each colored regular and action card, except for the zero card, which only have one per suit. There are four "wild" and "wild draw four" cards each, producing a total of 108 cards. In older versions, only the 6 is underlined to distinguish it from the 9, which is not marked; newer versions have both the 6 and the 9 underlined to further distinguish the two ranks.

Before playing, a dealer must be selected. This is accomplished by drawing cards. The person with the card of the highest face value is the dealer. Only number cards are used for this purpose. Any other cards are then put back into the deck.

After the dealer has been selected, seven cards are dealt to each player, and the top card of the stock is exposed to start the discard pile. If the exposed card has a special ability, it is treated as if the dealer played that card, and the special effect occurs (i.e., skip, draw two, reverse, or wild). If the exposed card is a wild draw four, however, it is returned to the deck and the next card is exposed. Play begins with the person to the left of the dealer, i.e. clockwise.

At each turn, a player may play a card from their hand that matches either the color or rank (or both) of the top exposed card, or play a wild or wild draw four.

If the player doesn't have a card to match the one on the DISCARD pile, he/she must take a card from the DRAW pile. If the card picked up can be played, the player is free to put it down in the same turn. Otherwise, play moves on to the next person in turn.

Penalties
From the official Uno rules:

A player who forgets to say "uno" after his/her second-to-last card touches the discard pile, but remembers (and shouts "uno") before any other player challenges them, is safe and is not subject to the penalty. When another player calls out "uno", the player with one card must draw 2 cards.
Players who make card-play suggestions to the other players must draw 2 cards from the draw pile.
If a player plays a wrong card and it is noticed by any of the other players, he or she must take the card back and take 2 extra cards from the draw pile. Play continues with the next person in turn.
If a Wild Draw Four card is played illegally (that is, if the player holds a matching color to one on the discard pile) and the person who plays it is challenged, the hand must first be shown to the player who has made the challenge. If the Wild Draw Four card has been played illegally (if the player calls the challenge and if it works for you), the offending player must draw 4 cards. If the card has been correctly played (if the player calls the challenge and if it works against you), the challenger must draw 4 cards plus an additional 2 cards for a total of 6 cards. The challenge can only be made by the player who is required to pick up the 4 cards after the Wild Draw Four card is laid.

Strategy
A basic strategy involves playing the legal card with the highest point value. This is a simple way to minimize points held in the hand at the end of the round, but fails to account for the utility of holding wilds and draw fours near the end of the game.

It is important to note that playing a 0 is more likely to prevent the color from changing than playing any other card of a given color.

Little has been published on the optimal strategy for the Uno game. Simulations of Uno games may shed some light on the matter, but the game solution is likely to be very complex. This is because attempts to reduce point count in the player's hands can be "read" by other players if too transparent. This information can be exploited by other players, and it follows that a mixed strategy may be more appropriate.

Some work has been done into the psychology of Uno as it relates to individual and group behavior. Players may exhibit physical tells, in which a subtle, often repeated, visual cue inadvertently reveals their state of mind during a game. Alternatively, they may change their playing style; switching from an aggressive card-shedding strategy to a more subdued one, or vice versa.

Alternatively, the player may choose not to play a playable card from his/her hand. If so, the player must draw a card from the DRAW pile. If playable, that card can be put down in the same turn, but the player may not use a card from the hand after the draw.

After playing a single card or drawing, the next player clockwise takes a turn, or counter-clockwise when a reverse is in effect. If the stock is emptied, the discard pile is shuffled and turned over to replenish the stock.

When a player plays down to only one card, that player is required to yell "Uno" to warn other players. If the word is not said, and another player points out the error, the original player is busted and has to draw two cards from the DRAW pile. The hand is over when one player has discarded all of their cards.

After a player plays all their cards, the other players count the number of points pertaining to the values of the cards in their hands. Number cards are face value, colored special cards worth twenty, and wilds worth fifty. The first player to go out receives points for the cards left in his/her opponents' hands. The first person to reach a certain point value (officially 500) wins.