The androgen receptor (AR) is the key regulatory element of androgen signaling in the cell. It mediates action of androgens and is therefore essential for growth, function and differentiation of the human male urogenital tract. Genetic alterations in the AR gene may cause impaired development resulting in androgen insensitivity syndromes (AIS) or in neurodegenerative diseases like Kennedy syndrome. Besides the crucial role in the process of virilization during embryogenesis and puberty, the AR also plays an important role in the adult man as the intracellular mediator of androgen action. Androgen withdrawal and/or AR blockade is the main choice of treatment of nonorgan-confined prostate cancer. Unfortunately, this treatment is only palliative and a majority of these tumors recur and progress to an androgen-independent and therapy-resistant stage. Recent findings gave new insight into the molecular structure and function of the AR and improved our understanding about prostate cancer progression, consequently resulting in the development of novel treatments. It has become evident that the AR is a nuclear transcription factor that can be activated ligand-dependently by androgens as well as ligand-independently by other hormones and various growth factors, respectively. Moreover, it was shown that the interaction of the AR with other proteins of the intracellular signal transduction cascade may promote prostate tumor growth. This review will summarize the most important findings about the AR and the androgen signaling pathway to improve the understanding of prostate diseases and novel treatment strategies that may be useful in the clinic.