We propose and test a model of loyalty in games. Players can mutually maintain loyalty by working towards a common goal that is pareto-superior to any Nash equilibrium without it. Loyalty imposes a psychological cost on defecting in an ongoing cooperation, which is thus sustained. We distinguish loyalty from reciprocity and explain how it complements guilt aversion with two dynamic games from a field experiment conducted in a Pakistani factory. The evidence supports the validity of loyalty, which has a stronger effect within than between groups.