Plasma membrane of cells is the principal physical barrier limiting protein and amino acid (AA) movement between different metabolic compartments in an organism. Control of these movements is now recognised to be important for overall control of whole-body protein metabolism. 1 - 5 AA transport processes fulfill a number of specialised but essential body functions alongside their basic role in supplying cellular AAs for protein synthesis and cell metabolism. These include absorption/reabsorption 2of AAs (from intestinal/renal lumen, respectively), 6 - 8 control of neurotransmission (re-uptake of A A transmitters from synaptic cleft), 8 - 10 and inter-organ exchange of carbon and nitrogen. 1 , 2 The most quantitatively important sites of AA transfer in the human body are likely to be skeletal muscle, the kidneys, and tissues bathed by the splanchnic circulation (notably the liver and small intestine). 1 , 2 , 11 Dietary protein is hydrolysed to small peptides and AAs within the intestinal lumen; although peptide transport represents a considerable proportion of the total amino-N uptake across the brush-border membrane, intracellular peptide hydrolysis means that the overall trans-epithelial movement of amino-N is almost entirely in the form of AAs. 1 , 2 , 12 In this article, we provide a brief overview of the mechanisms involved in AA movements across cell membranes before describing methods available for their study, focusing on techniques applicable to in vivo or intact tissue/organ investigations.