This chapter describes the essential problem that has required such persistent attention and discusses certain of the theoretical attempts to deal with it. Several generations of theorists concerned with associative learning have wrestled with issues of "stimulus selection". "Associative learning" is a very general abstraction. It may be said to have occurred whenever an organism acts as though it had acquired some knowledge of the relationship between two or more stimuli. The laboratory investigation of associative learning suggests a more specific characterization of the problem. In Pavlovian conditioning the sequence of stimulation the animal receives is not dependent upon the subject's choice behavior but is under more prescribed experimenter control. Stimulus selection is manifest in the change from presolution to solution performance in selective learning, in the Pavlovian phenomena of overshadowing and blocking, in the overlap problem in discrimination learning, and in the shift in stimulus control that is sometimes referred to as "spontaneous configuring".