Gynecomastia is also called enlarged male breasts or man boobs. It is a benign enlargement of breast glands. Both men and women have breast glands. While in women they are very well developed, in men they are only rudimentary. But not Always. Let me explain. During puberty, a boy undergoes many changes. Changes in voice and a sudden increase in height are external changes caused by the changing internal hormone balance. In girls, the breast glands begin to develop under the influence of rising female hormones. Sometimes in men, the transient imbalance in the hormonal levels may stimulate male breast gland growth. In up to 60 per cent of boys, there is a noticeable increase in the male breast tissue to varying extents. The growth of excess breast tissue ceases as soon as the hormone levels stabilise. And as the chest enlarges the glands flatten out and become imperceptible. However, in a small percentage of boys undergoing puberty, the glands develop to such an extent that they remain perceptible even in adult life. This remnant, small female breasts-like presence is called gynecomastia. Gynecomastia glands develop from breast tissue but they are not the same as normal breast glands. They are firm and fibrous. Enlargement of breasts on both sides is more common, it is called Bilateral Gynecomastia. Enlargement on one side only is called unilateral gynecomastia. The most common cause of gynecomastia is idiopathic. That means male breast development happens on its own as I have explained above (it is not your fault). This is also called Pubertal gynecomastia. Other causes of gynecomastia include diseases and medications. The breast gland grows in the presence of female hormones hence any disease which increases the levels of female hormones in a male can likely cause gynecomastia. All the diseases that result in low testosterone levels(decreased male hormones) may also result in gynecomastia. Also, all the drugs and medications that affect the normal balance of male hormones vs female hormones may also result in gynecomastia.