Testosterone, synthesized by Leydig cells, is essential for the development and function of the male reproductive system and for the regulation and homeostasis of major biological functions. Although fetal and adult Leydig cells both produce testosterone, these cells originate from different precursors and may be differentially affected by environmental chemicals. The objective of this chapter is to examine the impact of environmental chemicals on Leydig cell function, focusing on animal experiments involving exposures to phthalate plasticizers as these ubiquitous compounds have emerged as primary disruptors of testicular function. Recognizing the need for risk assessment studies that are relevant to humans, the chapter focuses on studies of acute and chronic effects of individual toxicants and of chemical mixtures on fetal and adult Leydig cells.