A strategy in a non-cooperative repeated game in which a player cooperates until the opponent is observed not to cooperate. The observation of non-cooperation triggers a switch to punishment of the opponent. For example, in a repeated prisoners’ dilemma the trigger strategy is to play {Don’t confess} provided the opponent plays {Don’t confess}, and then to punish by playing {Confess} in every subsequent round if the opponent is ever observed to confess. See also tit for tat.