Arsenic is categorized by the WHO as the most significant environmental contaminant of drinking water due to the prevalence of geogenic contamination of groundwaters. Arsenic and the compounds which it forms are considered to be carcinogenic. The mechanism of toxicity and in particular of carcinogenicity of arsenic is still not well understood. The complexity originates from the fact that arsenic can form a rich variety of species, which show a wide variability in their toxicological behavior. The process of biomethylation was for many years regarded as a detoxification process; however, more recent research has indicated that the reverse is in fact the case. In this book chapter we give a summary of the current state of knowledge on the toxicities and toxicological mechanisms of organoarsenic species in order to evaluate the role and significance of these regarding their adverse effects on human health.