Hypochondriasis has common features with a number of other mental disorders, its boundaries are not always easy to draw and it occurs frequently with several psychiatric conditions. This review examines the relationships between hypochondriasis and other psychopathology in two ways. First, it discusses the differential diagnosis of hypochondriasis by identifying both the similarities and differences between hypochondriasis and organic diseases, psychotic disorders, depression, panic disorder, obsessive-compulsive disorder, generalized anxiety disorder, specific phobia of illness and somatization disorder. The greatest overlap seems to exist between hypochondriasis and panic disorder, but hypochondriasis can still be distinguished from it, as well as from other disorders. The diagnosis of hypochondriasis is not warranted in the presence of psychotic features, but it can be made alongside diagnoses of most other disorders, provided that hypochondriacal features are not better explained by them. In the second part, the article reviews studies of the cooccurrence of hypochondriasis and other disorders. The literature reveals some inconsistencies and discrepant findings, which is largely due to different settings in which studies have been conducted and various other methodological issues. Still, it appears that hypochondriasis is more likely to co-occur with depression, panic disorder and somatization disorder than with other psychiatric conditions, suggesting a closer relationship with them. The implications of these findings for the conceptualization of hypochondriasis, its diagnostic status and classification are briefly considered.