The aim of this study is to explore the discussion and management of diagnosis and non-diagnosis (which occurs when a genetic diagnosis of the client's problems cannot be made) in the genetic counselling setting. Two aspects of this setting are key to the identification of these analytic themes: the relatively high frequency of diagnostic uncertainty, and the process of genetic counselling as an intertextual chain of communicative events. To explore the way in which diagnosis and non-diagnosis are managed in interaction I supplement my analysis of the spoken clinic with a description of the written communication that provides a record of that talk: medical notes and the client's follow-up letter. Within diagnosis and non-diagnosis, I explore a series of sub-themes including; the use of evidence (from professional and client perspectives), uncertainty, normality and temporality. These themes are explored by focusing on a range of interactional strategies including reported speech, contrast and hedging. The data for this research consists of audio-recordings of 18 genetic counselling sessions, covering a variety of genetic conditions and symptoms, for clients at different stages of establishing a diagnosis. Copies of the accompanying medical notes and follow-up letters are also collected. This thesis suggests that clients within genetic counselling are able to actively participate in talk about diagnosis. I suggest that talk about diagnosis can serve to acknowledge and manage the clients' everyday experiences of their personal situation. However, in the giving of non-diagnosis the need to establish a diagnostic label is foregrounded, and everyday experiences are invoked to promote this need, or manage the non-diagnosis. Finally, I see non-diagnosis in the genetic counselling setting as a flexible concept that can be regarded as a continuum with differing degrees of uncertainty. The clinic interaction allows participants to negotiate their position along this continuum.