Sulfonium compounds and their related salts are one class of charged sulfur compounds which play important rôles within biology. Those naturally occurring sulfonium compounds which are essential for biochemical processes within living systems have been summarised. Synthetic compounds, some of which are analogues of established pharmacological agents, possess a variety of biological activities and have been investigated for many potential therapeutic applications including antitumour and neurological properties. Recent developments within this field have been reviewed together with the adverse toxicological implications arising from the biological generation of reactive intermediates and subsequent enzyme interaction.