Psychiatric diagnosis and interventions are based on behavioral and experiential symptoms, codified especially in the Diagnostic and Statistical Manual of Mental Disorders (DSM) and the International Classification of Diseases and Related Health Problems (ICD). Mental health researchers, however, are intensively seeking to identify biological parameters – so-called biomarkers – that are supposed to supplement or replace these ‘descriptive’ diagnostic criteria. The PhD project empirically investigates this search for biomarkers with a special focus on mood disorders. Based on ethnographic fieldwork, qualitative interviews, and document analysis, I map related expectations, hopes and fears (e.g. the vision of ‘personalized psychiatry’) and delineate epistemic strategies enacted by various actors in the fields of genomics, proteomics and brain imaging. I assume that the search for biomarkers might significantly reconfigure mental disorders and may already be transforming the knowledge infrastructure of the psy-disciplines – even though this endeavour has largely failed so far.