We often hear that certain mental disorders are disorders of the brain, but it is not clear what this claim amounts to. Does it mean that they are like classic brain diseases such as brain cancer? I argue that this is not the case for most mental disorders. Neither does the claim that all mental disorders are brain disorders follow from a materialist world-view. The only plausible way of understanding mental disorders as brain disorders is a fairly modest one, where we label brain differences we find in mental illness as pathological based on their link to mental dysfunction. How many mental disorders will turn out to be brain disorders on this understanding is an empirical question.