Carrier-mediated membrane transport of small peptides is now realized to be a process of wide biological distribution, occurring not only in the small intestine and elsewhere in the animal body but also in bacteria, yeast, the mould Neurospora crassa, and probably in higher plants during the germination of seeds. The important features of peptide transport are outlined, and possible relationships between peptide transport and hydrolysis are discussed. Peptide transport is a stereochemically specific active process, and is independent of the transport of free amino acids. It is frequently, though not always, more rapid than the transport of amino acids. In the intestine, it is probably limited to dipeptides and tripeptides, but certain other animal cells and bacteria can take up larger peptides of seven or more amino acid residues. The ability to take up small peptides on a large scale is nutritionally important in some microorganisms, and might be of nutritional importance to the intact animal and to animal cells in culture. In the absorptive cells of the small intestine, and in Escherichia coli, peptide transport into the cells is followed by intracellular hydrolysis; transport and hydrolysis are quite distinct processes. Whether hydrolysis and amino acid transport can be coupled processes, or whether peptide transport and hydrolysis are different aspects of the same process, remains to be seen. This question is one of those where a close integration of studies of peptide transport with those of peptide hydrolysis should be particularly helpful.