Experimental studies in animal models have contributed significantly to our understanding of the pharmacokinetics of drug disposition in the maternal-fetal unit. This chapter has considered some of the factors that are important in determining the extent of fetal exposure to drugs that are consumed by the mother during pregnancy. A knowledge of the maternal-fetal pharmacokinetics of the drugs of abuse will help provide a better understanding of the limitations of the current methods used in perinatal drug screening and in the interpretation of the results.