Muscle phenotype is central to the quantity and quality of meat production. Quantity is a function of muscle hyperplasia and hypertrophy. Hyperplastic growth in utero is a function of myocyte proliferation and differentiation, and post-natal growth is primarily the result of hypertrophy of existing and replaced muscle fibres. Quality is a more complex trait and is assessed by a variety of objective and subjective parameters, such as colour, pH, tenderness, odour and juiciness. It is increasingly evident that fibre type composition is a major determinant of meat quality. Favourable meat traits, such as tenderness, have been found to associate with the greater abundance of oxidative fibres. Therefore, knowledge of the molecular events that affect hypertrophy (or atrophy), hyperplasia (or hypoplasia), and fibre type-specific expression is of fundamental agricultural importance.