Gynecomastia is a benign, abnormal, growth of the male breast gland which can occur unilaterally or bilaterally, resulting from a proliferation of glandular, fibrous and adipose tissue. Gynecomastia is characterised by the presence of soft, 2-4 cm in diameter, usually discusshaped enlargement of tissues under the nipple. It is estimated that this pathology occurs in 32-65% of men over the age of 17. Gynecomastia is a psychosocial problem and may lead to a perceived lowering of quality of life. The main cause of gynecomastia is a loss of equilibrium between oestrogens and androgens. Increased sensitivity for oestrogens of the breast gland, or local factors (e.g. an excessive synthesis of oestrogens in breast tissues or changes in oestrogen and androgen receptors) may cause gynecomastia. Also, prolactin, thyroxine, cortisol, human chorionic gonadotropin, leptin and receptors for human chorionic gonadotropin, prolactin and luteinizing hormone localised in tissues of the male breast may participate in the etiopathogenesis of gynecomastia. Usually three types of gynecomastia are distinguished: physiological, idiopathic and pathological gynecomastia. The latter is the consequence of relative or absolute excess of oestrogens. In this paper, frequent as well as casuistic causes of gynecomastia will be described. A diagnosis of gynecomastia is usually possible after a palpation examination. Ultrasonographic, mammographic or histopathological examinations are useful in aiding diagnosis. The five degree scale devised by Tanner and Marshall is useful in estimating disease progression.