In this thesis I provide an account of the unpleasantness of pain. In doing this, I shed light on the nature of pain and unpleasantness. I propose to understand the unpleasantness of pain based on the determinable-determinate distinction. Unpleasantness is a determinable phenomenal property of mental states that entails badness. I propose that an unpleasant pain experience has two phenomenal properties: i) the phenomenal property of being a pain, and ii) a phenomenal determinate property (u1, u2, u3, etc.) of the (...) unpleasantness determinable. According to this theory unpleasant pains feel bad, and this explains why we are motivated and justified in avoiding them. This explains, for example, why we are motivated and justified to take painkillers. This theory allows us to account for the heterogeneity of unpleasantness, i.e., we can explain how different unpleasant experiences feel unpleasant even if they feel so different. -/- The thesis is organised into seven chapters and divided by three main themes: i) what the unpleasantness of pain consists in, ii) how we can account for the great phenomenal diversity among experiences of unpleasantness, and iii) which cases suggest that there could be pains that are not unpleasant. Broadly, the first two chapters deal with the first theme, where I analyse two reductive accounts of unpleasantness: the content theories and the desire theories. I deal with the second theme in the third and fourth chapter, where I analyse different theories that try to account for the phenomenal property of unpleasantness. In the fifth and sixth chapter, I focus on the third theme, where I consider different cases that suggest the existence of pains that are not unpleasant. In the final chapter, I offer a conclusion of the three main themes by providing my own view on the unpleasantness of pain. (shrink)