This article describes a program of research applying Ecological Momentary Assessment (EMA) methods to study relapse to cigarette smoking, with a particular focus on the role of negative affect (NA) and self-efficacy (SE). Day-to-day changes in mood and stress did not predict lapse risk, but more proximal changes in affect were associated with lapses: Many lapses were marked by intense NA and by NA increases in the preceding hours. Individual differences in baseline SE predicted lapse risk, but daily SE was relatively stable during abstinence and did not influence lapse risk. However, lapses resulted in immediate drops in SE, and day-to-day changes in postlapse SE predicted progression to relapse, even after accounting for concurrent smoking. SE showed momentary drops associated with NA, but only among smokers with low baseline SE. Individual differences in baseline SE were only expressed situationally under conditions of NA. The findings highlight the importance of dynamic changes in background conditions and in immediate states as important influences on lapses and relapse and also suggest the importance of considering person by situation interactions.