Austen Clark offers a general account of the forms of mental representation that we call `sensory'. To sense something, one must have some capacity to discriminate among sensory qualities; but there are other requirements. What are they, and how can they be put together to yield full-blown sensing?Drawing on the findings of current neuroscience, Clark proposes and defends the hypothesis that the various modalities of sensation share a generic form that he calls 'feature-placing'. Sensing proceeds by picking out place-times in or around the body of the sentient organism, and characterizing qualities (features) that appear at those place-times. Such feature-placing is a primitive kind--probably the most primitive kind--of mental representation. Once its peculiarities have been described, many of the puzzles about the intentionality of sensation, and the phenomena that lead some to label it 'pseudo-intentional', can be resolved. The hypothesis casts light on many other troublesome phenomena, including the varieties of illusion, the problem of projection, the notion of a visual field, the location of after-images, the existence of sense-data, and the role of perceptual demonstratives. A Theory of Sentience will interest anyone interested in the topics of sensation, representation, or phenomenal consciousness.