The main body of this work contributes to an understanding of the development and growth of skeletal muscle in a range of Vertebrates from fish to pigs. Particular emphasis is paid to the contribution of numbers and types of muscle fibres to overall muscle growth and ultimate mass, and also to the mechanisms whereby factors such as nutrition in mammals and temperature in fish may affect these parameters. The work is divided into three main sections. The first section covers aspects of prenatal mammalian development including myogenesis and placentation. Muscle develops as two populations of muscle fibres. Primary myofibres form first and this is followed by the formation of a larger populations of secondary fibres. Restricting maternal nutrition may compromise the formation of secondary fibres but not primaries. Studies on the placenta and on levels of specific factors, e.g. insulin-like growth factors, has given some insight into the mechanism of nutritional effects on muscle fibre development. Nutritional experiments have highlighted energy levels in the earlier stages of gestation as most critical in the development of muscle fibre number. This finding has been developed in pig experiments which have shown that extra feed in early gestation can produce piglets with more secondary fibres at birth and which grow faster and more efficiently to slaughter. The second section incorporates work on postnatal mammalian muscle. Studies, on pigs in particular, have shown that primary fibre number relates more to genotype that does secondary fibre number. Total muscle fibre number correlates with some parameters of carcass leanness and with postnatal growth rate and feed conversion efficiency. The influence of factors such as nutrition, dwarfism, obesity and sex on aspects of muscle growth and muscle fibre types has been studied as well as the functional adaptation of muscle metabolism in different species. The third section includes work on fish muscle development and growth in a range of species.