This book chapter presents selected aspects of arsenic biochemistry, some of the effects of arsenicals on biochemical endpoints, three possible modes of action (MOA) (binding, oxidative stress and DNA methylation) for arsenic causing human cancer, several of the connections between arsenic and carcinogenicity (epidemiological studies, animal studies and the use of arsenic in the treatment of leukaemia) and finally some sources of information for newcomers to this large and complex research area. This chapter focuses more on individual speciated arsenicals and the individual proteins or other cellular targets of arsenicals rather than more descriptive studies of adverse animal health effects or pathological endpoints. The general order of the chapter is from smaller to larger biological systems. Entire books [1-3] and comprehensive review articles [4][5][6][7][8][9] have been published on the subject of arsenic. Therefore, in many cases this material is deliberately not included in this chapter if the material is well presented elsewhere.