Depression is one of the most important and common mood disorders. This study aimed to present a clear picture of the prevalence of depression in studies conducted in Iran. In this systematic review study, all published relevant studies were searched in credible scientific databases using standard key words. Repeated cases and those outside the time span of this study (1996 to 2011) were excluded. Articles were examined in terms of quality, and irrelevant ones were excluded. Eventually, 145 articles were selected for this systematic review. Fifteen different tools had been used in these articles, mostly standard tools such as Beck's and general health questionnaires. Beck's inventory indicated the prevalence of depression as 52.12% in students, 37.22% in the general population, 33.45% during postmenopausal period and 27.62% during pregnancy. General health questionnaire revealed the prevalence of depression as 12.02% in students, 19.46% in general population. For the older adults, Geriatric Depression Scale (GDS) played an important role. GDS revealed the prevalence of depression in older adults living at home as 57.58%, and in those living in nursing homes 81.85%. Because of its ubiquitous features, Beck's inventory had been used, which showed overall prevalence of depression 42.59%. Considering high prevalence of depression in Iran, greater attention to depression and preventive measures are hugely important.