Efflux is by far the most common means of arsenic detoxification is by methylation catalyzed by a family of As(III) S-adenosylmethionine (SAM) methyltransferases (MTs) enzymes designated ArsM in microbes or AS3MT in higher eukaryotes. The protein sequence of more than 5000 AS3MT/ArsM orthologues have been deposited in the NCBI database, mostly in prokaryotic and eukaryotic microbes. As(III) SAM MTs are members of a large superfamily of MTs involved in numerous physiological functions. ArsMs detoxify arsenic by conversion of inorganic trivalent arsenic (As(III)) into mono-, di- and trimethylated species that may be more toxic and carcinogenic than inorganic arsenic. The pathway of methylation remains controversial. Several hypotheses will be examined in this review.