Marine organisms can contain significant amounts of arsenic (As), which is an initially alarming observation with regard to human health. Extensive speciation studies have provided evidence for the presence of As in seafood in a variety of compounds. Although it is assumed that As is passed up the food chain via diet, and although the abundance of the several arsenicals in the marine ecosystem is well documented by now, the understanding of its exact origin and metabolical pathways remains hypothetical. Also, the origin of large differences in As concentration among fishes, both interspecies and intraspecies, remains to be elucidated. A reassuring finding is that the majority of the As in seafood is present in forms that are believed to be nontoxic. The fraction of toxic As generally remains reasonably low and, thus, is not believed to pose a direct threat to human health upon consumption of seafood. Nevertheless, formal norms for permissible levels of 58contamination are still ambiguous. Extensive research has also provided information about the fate of both the toxic and the nontoxic As compounds in the human body upon ingestion. Even though the majority of the ingested As is excreted in the urine, either unchanged or methylated, some cautions regarding possible health effects should be considered.