Teratology is the study of anatomical and physiological abnormalities, commonly known as birth defects. If an embryo is exposed to a harmful substance, or teratogen, during the critical period of development, an ensuing malformation may occur. These malformations and their associated mechanisms are studied and analyzed in laboratory animals in order to prevent them from occurring in humans. Rodents, such as rabbits, rats, and mice, have commonly been used in such studies because of their similarity to humans. In 1959, James G. Wilson designed, developed, and tested a protocol on how to observe and analyze structural malformations in rodent fetuses, which included external examination, skeletal evaluation, soft tissue analysis, and data collection/analysis. Although many years have passed since Wilson created this protocol, it is still widely used to this day, and only minor changes have been made to his instructions such as the chemicals used in the experiments and also the analysis of the experimental data. While only minor modifications have been made to this protocol since its beginning, major advances have been made in the dissemination of teratology information to the public such that information is now available through the Internet--information including the identification of an increasing number of teratogens and the understanding of the pathogenesis as it relates to the etiology of birth defects. Despite these advances, however, there has been little decrease in the overall incidence of major birth defects, although significantly improved reporting and ascertainment of birth defects must be factored into the equation in determining birth defect rates. Future birth defect prevention may be based on the understanding of individual genomes and pharmacogenomics, and as the interaction between teratogenic and genetic factors is better understood--with the hope that the incidence of both chemically induced and genetic defects will one day be substantially reduced.