Presents the obituary of William D. Timberlake (1942-2019). William David Timberlake contributed to psychology in many ways. He is best known for his behavior systems approach to learning and for his disequilibrium theory of reinforcement. He also made important contributions to behavioral economics, circadian rhythms, time horizons, adjunctive behavior, and contrast effects. In these pursuits and others, Timberlake brought clarity to vexing theoretical problems through careful attention to the species-typical characters of the subjects and how these interact with the specifics of apparatus and procedures. He focused on understanding the animals as animals (what he called a theromorphic approach). Timberlake embraced the complexity of behavior, its organization, and its relation to the body as part of a whole, functioning organism (PsycInfo Database Record (c) 2020 APA, all rights reserved).