This chapter is about theories of work-related stress. Of course, throughout this Handbook, stress-related topics are discussed. However, in order to understand different theories and to give them a sense of time, place, and meaning, we attempt to explore them against the changes in how stress has come to be defined. The importance of exploring stress theories in this way lies in the way it gives a sense of history: of why different theories prevailed (Cooper, Dewe, & O’Driscoll, 2001), whether they are “worthy of the intellectual resources focused on them” (Kaplan, 1996, p. 374), whether they adequately express the nature of the experience itself (Newton, 1995) and, despite the knowledge and understanding they have provided, whether they are still capable of expressing “the stress of the stress process” (Lazarus, 1990, p. 4). We also explore whether we can distil from them what should now become the organizing concept of the future around which such theories should focus. Liddle (1994) describes an organizing concept as one with “sufficient logic and emotional resonance to yield systematic theoretical and research enquiry that will make a lasting solution” (p. 167). Finally, we explore the different theories in terms of how they have influenced our measurement strategies, where our current methodologies are taking us, what this means for understanding the richness of the stress experience, and the type of evidence they provide in terms of work stress and well-being. However, this chapter does not review all the different theories of stress. In order to explore how they have evolved, we have selected a number that best express this evolutionary process, although all theories have an evolutionary element to them. A comprehensive review of stress theories can be found in Cooper (2000). This book is as “a compendium of theory rich in diversity and range” (p. 4) emphasising not just the need for theories to capture the essence of the work experience itself, but also help us as researchers fulfil our moral responsibility to those whose working lives we study. This chapter begins by first exploring the evolutionary milestones in the way stress has been defined. It then uses this as the context for exploring the development of selected stress theories. The chapter concludes by exploring what this means in terms of our understanding of work stress, those elements that should now be reflected in our theories of stress and the issues we now need to consider as researchers and practitioners.