Omega-3 polyunsaturated fatty acids (EPA, DHA, ALA) support brain development, joint health, skin and coat condition, and anti-inflammatory pathways. The best sources are cold-water fish oils (salmon, sardine, menhaden) which provide preformed EPA and DHA. Plant-based omega-3 (ALA from flaxseed) requires conversion to EPA/DHA, which dogs do inefficiently and cats barely at all. Look for fish oil specifically named in ingredient lists.