The symposium describes studies on the short- and long-term effects of fetal and early postnatal exposure to hormone mimics. This work can inform the public about two important issues: the use of animals in research and how mechanistic in vitro methods can reduce animal use; and the concern about the adverse health consequences to humans by exposure to endocrine disruptors from foods and some plastics used to contain these foods. Early exposure to naturally produced or injected testosterone has long-term consequences in animals. It is becoming clear that the presence of testosterone, estrogen, or their mimics during early development can also “organize” a variety of tissues to be more or less responsive to these hormones when they are activated at puberty and throughout adulthood. Such consequences have become more compelling with the recent discovery of multigenerational effects of endocrine disruptors via epigenetic mechanisms. Speakers will discuss the outcomes of early exposure to endocrine-active substances on brain development, behavior, reproductive organ development, fertility, precancerous conditions of the prostate and breast, and their relevance to humans. The importance of testing for these agents, including the development of non-animal methods, will also be addressed.