This chapter focuses on attempts to understand avoidance learning. The original experiments were first performed in the St. Petersburg laboratory of Pavlovs arch-rival, Vladimir Bekhterev. Researchers there arranged that, if when a signal was given, a dog failed to flex a leg, a shock would be delivered; if the leg was flexed in time, the shock was avoided. In general, these dogs learned quickly to flex the target leg as soon as the signal was given. It took over 30 years before a widely accepted explanation was developed of how the absence of an event could promote learning. A key contribution was the two-factor theory developed by Hobart Mowrer. His studies of avoidance learning and those that followed, mainly by Richard Solomon and his students, laid the foundation for breakthroughs in the study of associative learning in the late 1960s. The chapter also describes research on punishment.