Burnout is hot topic in public discourse, with members of the general public quick to self-diagnose as suffering from the syndrome. However, there is a lack of consensus among scientists and practitioners as to how burnout should best be conceptualised and whether/how it should be diagnosed. This thesis reports five studies examining self-diagnosed burnout and how it should best be defined, measured and distinguished from clinical depression. The literature review presented in Part I critiques the currently promulgated three-factor conceptualisation of burnout and highlights the inconclusive research findings regarding burnout’s overlap with depression. Part II contains three studies designed to re-define the burnout syndrome. In Study 1, qualitative and quantitative analyses were used to identify key syndromal features of self-diagnosed burnout. In Study 2, a series of bifactor analyses was undertaken to derive a new definitional model and preliminary measure of the syndrome. In Study 3, mixture modelling was used to examine whether the new burnout definition was best modelled dimensionally or categorically. The results of Part II indicated that burnout as experienced by the general population is characterised by several symptoms beyond the traditional triadic symptom model, and that categorical differences in burnout may exist between those with and without a history of mental illness. Part III contains two studies undertaken to evaluate the degree of overlap between burnout and depression. Study 4 assessed for qualitative differences between burnout and depression experienced by participants who reported having experienced both states. Study 5 compared participants with self-diagnosed burnout to participants with clinically-diagnosed depression across several symptom and causal variables. The results of Part III indicated that the new definitional burnout model derived in Part II showed poor differentiation overall between burnout and depression, but that several other phenomenological, symptom and causal differences are likely to exist between the two states. The results of Study 5 also suggested that burnout overlaps more with non-melancholic than melancholic depression. When taken together, as discussed in Part IV, the studies in this thesis extend knowledge of how self-diagnosed burnout should be defined and illuminate how burnout both converges with and diverges from clinical depression.