Despite the fact that formaldehyde exposure may cause reproductive and developmental toxicity, as suggested by evidence from both human and experimental animal studies, the current understanding of the likely mechanisms of action (MOA) is very limited. To date, few human studies have been designed to investigate possible formaldehyde MOAs, though hypotheses have been generated from limited preliminary results obtained in recent animal studies. This chapter describes mechanisms by which formaldehyde is proposed to induce reproductive and developmental toxicity, including through genotoxicity, oxidative stress, apoptosis, DNA methylation, and disruption of the activity of proteins, enzymes and hormones important for the maturation of the male reproductive system. It should be noted that most of the proposed mechanisms are hypothetical and require validation, particularly in reproductive systems.