Maintaining amino acid (AA) homeostasis is necessary for organisms. To achieve this, organisms have evolved various signalling pathways regulated by sensing general or specific AA levels. Recently, advances in genetic and dietary manipulation have shed light upon how these AA signalling pathways regulate organismal physiology, metabolism, behaviour, and lifespan. However, elucidating the detailed mechanisms by which each AA is sensed and influences an animal's life is challenging. In some model organisms such as Drosophila melanogaster, chemically defined diet has been developed to manipulate single nutrients, which enables us to study the organismal response to dietary restriction of particular AAs. In this review, we aim to discuss the latest findings on animals' responses to dietary AAs, with a focus on recent studies in Drosophila.