The composition of human milk can be affected by the diet consumed by the lactating woman. The influence of the maternal diet on milk composition varies in magnitude between nutrients; for some nutrients no effect at all has yet been documented. Concentrations of fatty acids, fat-soluble and water-soluble vitamins in milk are, in general, significantly affected by the levels of these nutrients in the diet. Protein concentration may be affected under some conditions, although the magnitude of this change appears relatively limited. Lactose, mineral, trace element and electrolyte concentrations seem comparatively resistant to varying maternal intakes. Although there has been significant progress in this research area in the past decade, many early studies are difficult to interpret due to limitations in the study design and analytical methods. This review demonstrates a distinct need for carefully controlled studies on the effects of both nutrient deficiencies and supplements on milk composition. Interactions among nutrients, homeostatic mechanisms and energy balance (weight loss) are factors that need to be studied further. Information from such research will suggest strategies for nutrition intervention in areas of poor nutrition and provide dietary guidelines for lactating women.