Since the early eighties, there has been a growing interest in the descriptive epidemiology of obsessive-compulsive disorder (OCD). In this narrative review, the authors describe the findings of a number of studies that employed selected instruments, such as the Diagnostic Interview Schedule, the Composite International Diagnostic Instrument, and the Schedule for Affective Disorders and Schizophrenia, to ascertain the prevalence and incidence rates for OCD in several different countries. We noted that there is a great heterogeneity of findings and that the potential reasons for this variability include not only the intrinsic characteristics of the population under study but also extrinsic factors (i.e., the several methodologically-informed decisions that are to be made before undertaking such investigations, such as the adoption of a specific diagnostic instrument). In order to further the knowledge on the epidemiology of OCD, it would be worthwhile to establish a global consensus regarding a standard assessment package for OCD, to produce more cross-culturally valid versions of the key research instruments, and to conduct studies specifically aimed at comparing the sociodemographic, clinical and prognostic aspects of OCD across different countries.