Food is a vehicle for numerous undesirable substances. Some are naturally present in plant foods without causing many problems. These include alkaloids (e.g. solanine, caffeine and symphytine), psoralens, vasopressor amines, cyanogenic glycosides, phytoestrogens and lectins. Animal foods containing toxins are more unusual, most being fish (e.g. puffer fish containing tetrodotoxin), histidine in mackerel etc. and the various shell fish toxins. Toxins secreted by fungi parasitic on peanuts, cereals etc. are a major problem, as are those secreted by food borne bacteria, e.g. Clostridium botulinum and Staphylococcus aureus. Various foodstuffs are well known for the allergenic proteins they contain. Although most consumers are unaffected the allergens in nuts (especially peanuts), cereals, milk and eggs are a major hazard for susceptible individuals. Agricultural practices are a major source of food contaminants with toxic potential. Authorities around the world establish maximum safe levels for pesticide residues and monitor levels in food on sale. Some of the UK data obtained is summarized here. Residual nitrates, antibiotics and hormones are also potential hazards generated by agriculture. Toxic metal residues, e.g. lead, mercury and aluminium, can arise during food production. Other toxins are generated in heated food, e.g. polyaromatic hydrocarbons and acrylamide, or during packaging, e.g. bisphenol A. The chapter concludes with a list of specialist books and review articles for further study.