The endocrine system is a complex system that relies on the functioning of numerous glands. Each of these glands secretes hormones that are distributed throughout the body via the blood and lymphatic systems. This hormonal system is essential for maintaining all biological balances. Any substance foreign to the body that is capable of disrupting this functioning is called an "endocrine disruptor" (ED). For many years, the issue of EDs has been a concern for researchers, the general public, and regulatory agencies. Significant efforts have been made to better understand their mechanisms of action. The best-characterized mechanisms to date are interaction with hormone receptors and disruption of hormone synthesis and metabolism. However, as our understanding of EDs’ mechanisms of action advances, new questions emerge as fresh challenges. These new issues include the effects of low doses, mixtures of substances, and windows of susceptibility to exposure, particularly during fetal development. Research on these issues will have an impact not only on our understanding of modes of action, but also on the prediction of the occurrence of these toxic effects and on the regulatory framework governing the use of chemicals. (R.A.)