Sensing of nutrients is foundational for life and allows for cellular adaption to environmental stress. The aim of this review is to provide a basic background and overview of new data on cellular nutrient sensing.We focus on sensing of energy by adenosine monophosphate-activated protein kinase, sensing of amino acids via general amino acid control nonderepresible 2 and regulation of cellular metabolism via mammalian target of rapamycin pathways. Over the last 5 years, new data has elucidated how these pathways are both interconnected and regulated by amino acids.Regulation of these pathways is complex and tightly orchestrated in order to provide unified cellular responses that meet environmental demands. Amino acids provide key cues for the regulation of mammalian target of rapamycin pathways, in particular.