Adaptation, the desensitization to persistent changes in environmental conditions, is present throughout biological sensory mechanisms. Not surprisingly, it has been an active area of research to systems biologists. Here, we consider some of the models proposed to account for adaptation as well as the experiments used to motivate and validate these models. We discuss some salient features of these models including robustness, deadaptation, transient responses, and the response of these systems to more complex temporal stimuli. While most of these models have been used to study chemoattractant-induced responses in bacteria and amoebae, the system-theoretic issues associated with these systems are of importance in a broad spectrum of biological systems.