This chapter is an attempt to formulate ideas about the operations performed by physiological mechanisms, and not merely a discussion of the physiological mechanisms of sensory pathways. It presents three hypotheses regarding the purpose of sensory relays. The first one is the “password” hypothesis, which posits that, since animals respond specifically to specific stimuli, their sensory pathways must possess mechanisms for detecting such stimuli and discriminating between them. The second hypothesis is the fashionable one that relays act as control points at which the flow of information is modulated according to the requirements of other parts of the nervous system. Finally, the third hypothesis theorizes that reduction of redundancy is an important principle guiding the organization of sensory messages and is carried out at relays in the sensory pathways.