The purpose of reproductive toxicity studies is to evaluate all effects resulting from paternal or maternal exposure that interfere with conception, development, birth, and maturation of offspring. In 1966, the US Food and Drug Administration (US FDA) published guidelines for a three-segment study for drug testing to examine adverse effects on fertility and pregnancy. Three segments were proposed: Segment I, Study of Fertility and General Reproductive Performance, to provide information on breeding, fertility, nidation, parturition, neonatal effects and lactation: Segment II, Teratological study, to provide information on embryo toxicity and teratogenicity: and Segment III. perinatal and Postnatal Study, to provide information on late fetal development, labour and delivery, neonatal viability, and growth and lactation. The classic guideline is still used to this day with only monor modification throughout the world. In the present review, the principles and methods of reproductive toxicity studies are discussed with special attention given to scientific issues.