A theory of consciousness must specify which mental states can be conscious and the conditions under which they become conscious. Recent research in psychology and neuroscience can settle both of these questions. Evidence strongly suggests that consciousness arises at a specific stage of processing in our perceptual systems, and it arises when and only when we are paying attention. Developing these proposals requires an account of what attention is, both psychologically and biologically. The talk presents a account of how attention changes cellular activity within perceptual hierarchies, and replies to recent experiments which purport to show that attention and consciousness are dissociable. The theory is also compared to others in philosophy, psychology, and neuroscience.