Parents often bring their 10-year-old boys to the clinic to see breast enlargement, asking why boys have breast lumps and slight tenderness, worrying if there is something wrong or a tumor? To answer these questions, we must first understand the process of puberty development in boys: Generally speaking, the development of boys begins with the enlargement of the testicles and accelerated growth, and with the development of secondary sexual characteristics, the secretion of the androgen “testosterone” increases, pubic hair and acne appear, and gradually begins to change voice, and finally completes the development process. During this developmental process, when testosterone production increases, there is also an increase in the production of the transient estrogen estradiol, which leads to breast lumps and tenderness in boys. As development progresses and testosterone becomes absolutely dominant in boys, breast lumps will gradually disappear. This is a normal phenomenon in the development of boys, parents should not be alarmed by this situation.