Formaldehyde is a known human carcinogen for nasopharyngeal, sinonasal, and lymphohematopoetic cancers such as myeloid leukemia. In this chapter, the major epidemiologic studies that have demonstrated a causal link between formaldehyde and cancers are briefly discussed; and, the history of the classification of formaldehyde as a carcinogen is detailed. Additionally, the controversy surrounding formaldehyde’s carcinogenicity, particularly for formaldehyde-associated leukemia is deliberated; and, the major arguments against the formaldehyde–leukemia association are specifically outlined and briefly addressed one-by-one. Finally, this chapter is intended to serve as an introductory overview of the formaldehyde–leukemia association, which is further discussed in later chapters (Chapters 6–8) through a dissection of human epidemiologic studies and potential mechanisms of formaldehyde-associated leukemogenesis.