Most well-known warfare chemicals have similar molecular structures as the organophosphorus compounds. Percutaneous absorption and regional variation form the final barrier between the “outside” and the “inside” of the body where toxicity occurs. Percutaneous absorption is influenced by moisture, and certainly a soldier or civilian will sweat, and it does rain. Clothing also affects percutaneous absorption because material can act as both a barrier and a skin delivery system. Sweating, clothing, and skin absorption are thus interrelated. This chapter focuses on the determination of the percutaneous absorption of parathion when applied to naked human skin and uniformed skin with and without sweat and prediction of potential human absorption and lethality of the structurally related chemical warfare agent (CWA) VX. CWAs are easily and inexpensively produced and are a significant threat to military forces and to the public.