In psychiatry there is no sharp boundary between the normal and the pathological. Although clear cases abound, it is often indeterminate whether a particular condition does or does not qualify as a mental disorder. For example, definitions of ‘subthreshold disorders’ and of the ‘prodromal stages’ of diseases are notoriously contentious. Philosophers and linguists call concepts that lack sharp boundaries, and thus admit of borderline cases, ‘vague’. This overview chapter reviews current debates about demarcation in psychiatry against the backdrop of key (...) issues within the philosophical discussion of vagueness: Are there various kinds of vagueness? Is all vagueness representational? How does vagueness relate to epistemic uncertainty? What is the value of vagueness? Given the immense social, moral, and legal importance of demarcating the normal from the pathological in psychiatry, what are the pros and cons of gradualist approaches to mental disorders, that is, of construing boundaries as matters of degree? (shrink)