In recent years, considerable scientific interest has been devoted to amino acid supplementation and its role in regulating skeletal muscle metabolism in health, ageing and disease. This interest has, in part, stemmed from clinical evidence that traditional nutritional supplementation in patients is largely ineffective. In particular, this knowledge has prompted extensive research into the mechanisms responsible for amino acid stimulation of muscle protein metabolism in older adults with sarcopenia. It is well established that ageing diminishes muscle strength and size, contributing to a number of serious health problems such as an increased risk of falls and fractures. Although the reasons behind age-related sarcopenia are multifactorial and involve a complex interplay of hormonal and metabolic stimuli, it is clear that inadequate nutrition and inactivity are strong causative factors. Recently, the idea that elderly muscle is perhaps less anabolically sensitive to amino acids has been put forward as an additional causative factor. In light of this recent evidence, and the further suggestion that the effects of essential amino acids on skeletal muscle are independent of changes in insulin, newer studies are focusing on tailoring the optimal amount and proportion of amino acids needed to stimulate muscle protein synthesis most efficiently in ageing populations. Understanding the mechanisms of amino acid stimulation of muscle protein synthesis may also provide insight into the metabolic regulation of anabolism in skeletal muscle by key hormones such as growth hormone. In this article, we summarize current research concerning amino acids and skeletal muscle, particularly with regard to ageing and inactivity.