Koi Koi is one of the most played Hanafuda games in Japan. Hanafuda is a traditional Japanese set of cards depicting nature for each of the twelve months of the year.

You have to match cards from your hand and from the deck with cards of the same month in the field to capture them and create a Yaku (card combination that can be scored).

Once you have created a Yaku, you can stop and claim your points or say "Koi Koi" to keep going and try forming a more powerful Yaku.