Premature thelache is the development of breast tissue prior to the normal age of pubertal onset. it can arise as a result of a number of different etiologies. This chapter provides an overview both of the physiology of normal breast development and of the causes of early thelarche, including benign self-limited premature thelarche, gonadotropin-independent precocious puberty, gonadotropin-dependent precocious puberty, exposure to exogenous estrogens, and non-hormonal mechanisms. A careful history and physical examination is essential is essential in order to determine the proper diagnostic approach. It is important for the clinican to differentiate benign premature thelarche from other causes that can have deleterious consequences, such as true precocious puberty.