Certain long chain polyunsaturated fatty acids (LCPUFAs) are thought to be essential components of the nutrition of infants, including those prematurely born, in the sense that they cannot be synthesises by the immature organism and must therefore be supplied in the diet. Breast milk contains these substances, but many manufactured infant formulae do not. An absence of dietary LCPUFAs has been thought to affect the development of the brain and retina, possibly leading to abnormalties in cognitive and visual function. Considerable multidisciplinary research has been carried out to investigate this proposition. Diets free from LCPUFAs have been compared with supplemented formulae, or with breast milk. The conclusions from this research were critically examined by a group of leading paediatricians, nutritionists, experts in visual science and developmental behavioural scientists at a 'Dobbing Workshop' held in the United States in late February, 1997. Each of the Chapters was precirculated to the whole group, commented on before the Workshop, and then exhaustively discussed. The Chapters and Commentaries which are published here have therefore undergone a more extensive peer-review process than is usually the case.