We attempt to bring the concepts of pain, suffering, and anxiety into sufficient focus to make them serviceable for empirical investigation. The common-sense view that many animals experience these phenomena is supported by empirical and philosophical arguments. We conclude, first, that pain, suffering, and anxiety are different conceptually and as phenomena, and should not be conflated. Second, suffering can be the result--or perhaps take the form--of a variety of states including pain, anxiety, fear, and boredom. Third, pain and nociception are not equivalent and should be carefully distinguished. Fourth, nociception can explain the behavior of insects and perhaps other invertebrates (except possibly the cephalopods). Fifth, a behavioral inhibition system associated with anxiety in humans seems to be present in mammals and most or all other vertebrates. Based on neurochemical and behavioral evidence, it seems parsimonious to claim that these animals are capable of experiencing anxious states.