B. F. Skinner. Because of behaviorism's emphasis on the observable there were some things that they just didn't talk about. Behaviorists would not discuss memory, language, and other mental activities. At least they wouldn't be discussed as we would discuss them, with reference to mental events and thoughts that are unobservable by other people. They tried to explain these phenomena by constructing theories involving biological drives and habits. B. F. Skinner wrote a book about verbal behavior, i.e. language, and how it could be explained without any postulation of unobservable mental events.
Behaviorism dominated psychology for several decades, and many psychologists can still be considered behaviorists. However, for the most part psychology has turned away from the strict behaviorist viewpoint. The mass migration in psychology occurred in the 1960s with the occurrence of the cognitive revolution.
Of the beginnings of the cognitive revolution was Noam Chomsky's review of B. F. Skinner's book on verbal behavior, in which Chomsky's refuted Skinner's arguments very neatly. For example, Chomsky pointed out that the creativity in language use cannot be accounted for by behaviorist theories. That review is one of the benchmarks that led to the end of behaviorism. Also, humans learn language at about the same age, which is unusual if language learning is dependent solely on environmental factors.