Encounter cards (formerly known and capitalized as Challenge cards) are a category of cards, comprising the Attack, Negotiate (formerly known as Compromise), Morph and Retreat card types, that are used to decide encounters. They are played face down during the planning phase and revealed during the reveal phase. Each main player must play one encounter card per encounter; allies do not play encounter cards.
Most encounter cards are attack cards, which have numbers printed on them; these, in addition to ships, form a player's total strength. The second most common type of encounter cards are negotiate cards, which are used to make deals or receive compensation.