The main function of the Leydig cells is to synthesize testosterone, but growing evidence supports the idea that Leydig cells are important in regulating the testicular immune environment. As such, Leydig Cells: Structure, Functions and Clinical Aspects discusses Leydig cells serving as immunological sentinels in the testis. Next, the authors explore the commercially available immortalized Leydig cell lines (TM3, R2C, MA-10 and MLTC-1) derived from rat or mouse and the results of studies using these cells. Additionally, the effects of the most commonly used environmental contaminants on Leydig cell function are discussed in the context of the impaired of steroidogenic pathway. The results of several studies are presented which demonstrate that lycopene supplementation improves and maintaines the normal level of testosterone by improving the expression of StAR protein and steroidogenic enzymes in the Leydig cells of polychlorinated biphenyl exposed rats.