The currently developed and validated in vitro tests for female and male fertility and also for developmental toxicity are described and evaluated according to their potential use as screening or replacement alternatives to the established in vivo tests in reproductive and developmental toxicology. Alternative methods today can only be used to evaluate a few specific components of the integrated reproductive functions in both females and males. However, in the field of developmental toxicity testing there is a strong theoretical and empirical basis for the predictive power of in vitro screens using mammalian embryos as well as embryonic cells and tissues. Several of these assays have been validated or are currently undergoing validation in several laboratories and are > 80% concordant with in vivo results. Failure to achieve 100% accuracy reflects the inherent limitations of these systems, which are manageable, as the concordance rates are still good. The level of concordance suggests that these assays are adequate for screening purposes to complement traditional in vivo testing. The use of these assays as screens will save valuable in vivo testing resources for those compounds most likely to enter the market and to which people will be exposed.