Although the need for selenium in human and animal nutrition is well recognized, the question concerning the proper form of selenium for supplemental use is still being debated. Ideally, selenium should be supplemented in the form in which it occurs naturally in foods. Because the L-isomer of selenomethionine (Se-met) is a major natural food-form of selenium, synthetic L-Se-met or enriched food sources thereof such as selenium yeast are appropriate supplemental forms of Se for humans; for animals, DL-Se-met is acceptable. Ingested Se-met is either metabolized directly to reactive forms of selenium or stored in place of methionine in body proteins. Se-met metabolism is closely linked to protein turnover. At constant intakes in the nutritional range, tissue Se levels increase until a steady state is established, preventing the build-up to toxic levels.