This dissertation examines how bipolar disorder, a common and disabling psychiatric condition, is made relevant as a participants' concern in a site of massively consequential psychological business - the psychotherapy session. As its central thesis is the claim that the practices by which bipolar disorder gets done as bipolar disorder are invariably absent in most formal accounts of the disorder. In this regard, the dissertation provides an empirically grounded description of a range of discursive practices associated with the doing of bipolar disorder in psychotherapy. This is undertaken from a discursive psychological orientation that draws extensively from ethnomethodology, conversation analysis, and Wittgensteinian philosophy. Following a review of bipolar disorder as a diagnostic psychiatric category, consideration is given to alternate conceptualisations which suggest the category is constructed in-and-through complex socio-historical practices which are often occluded and considered irrelevant to the category's situated deployment. This notion is used to provide a more sustained examination of how one might 'get at' such practices in situ by way of conducting ethnomethodological and conversation analytically informed investigations. In consideration of how one might approach psychological categorisation practices in talk-in-interaction, a discursive psychological orientation is developed which stresses the social, public nature of psychological categories in use. The empirical materials examined in the dissertation are drawn from a corpus of audio recordings of seven 'naturally occurring' psychotherapy sessions involving a clinical psychologist and five clients for whom the category 'bipolar disorder' has demonstrable relevance. Practices examined include those relating to the production and recognition of what might count as a bipolar disorder 'symptom', the manner in which 'moods' operate as account production devices, and the methods by which psychological terms (such as 'thought' and 'feel') operate in-and-as situated practices involved in psychotherapeutic business.