This chapter focuses mainly on the effects of organotin compounds in various human and animal models and describes the research performed to elucidate the immunotoxic mechanism of action of organotin compounds. Both dibutyltin (DBT) and tributyltins (TBT) organotin compounds can cause atrophy of the rat thymus. Cooke et al. studied the lactational transfer of TBTC and DBT by analysis of the stomach contents of suckling pups. TBT levels were undetectable in all dose groups, and DBT levels were detectable in the highest dose group. Next to mammals, organotin compounds have been reported to affect the immune function of aquatic organisms. Several studies demonstrated that tributyltin oxide (TBTO) induces programmed cell death (apoptosis) in thymocytes. Toxicogenomics techniques offer the possibility to assess the effects of potential toxic components on many parameters including thousands of mRNAs, proteins, and metabolites, and processes such as imprinting of genes, alternative splicing of mRNAs, and mutagenesis.