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Summary Epidemiological research may offer a key to interpreting gender incongruence (GI) and identifying the medical needs of people with GI. Currently, it is difficult to estimate the real prevalence of GI due to the use of different definitions. The data on GI prevalence are influenced by case definition and by the different methodologies applied by researchers to assess cases. Estimations of prevalence therefore vary widely and are estimated between 0.1% and 1.1% among adults, with a male-to-female (MtF) – female-to-male (FtM) sex ratio of 1–6. For children, the literature reports a prevalence ranging from 1% to 4.7%, with a predominance of MtF for children and from 1.2% to 16.1% in adolescents, among whom FtM appears to be predominant. Studies based on access to gender clinics may underestimate GI prevalence while studies focused on self- or parent-reported data may overestimate the phenomenon. The literature describes high rates of affective and anxiety disorders (18–80%) and frequent personality disorders (20–70%), suicide attempts, suicide and self-harm among people with GI. These problems seem to improve after gender affirmation treatment. Some authors report a high prevalence of Autistic Spectrum Disorders (ASD) (6–14%) among youth with GI. Mortality rates, HIV infection and sexually transmitted diseases are high among the transgender population, probably related to lack of prevention. It is crucial to determine the true prevalence of GI in order to deliver adequate medical support. Future studies should be based on large multicenter cohorts or population-based surveys, and should adopt respondent-driven sampling while including individuals who do not seek services offered at gender clinics, to limit the selection bias.