In drug development, structure-activity relationships (SARs) are an integral part of the process of finding efficacious and non-toxic analogs, and in vitro test systems which detect the biological activity of a particular chemical class have found an important role in SAR research. In contrast, SARs are not a prime component of the hazard identification process for environmental agents, largely because the mechanisms of their toxicity are often unknown. Where SAR approaches have been applied to environmental chemicals, the objectives have been to identify 'bad actors' or to attempt to identify the mechanism of toxicant action. This presentation will review efforts to develop predictive SARs for the developmental toxicity of environmental chemicals and discuss them in relation to the role of SAR in the validation of in vitro test systems.