We discuss a peptide that targets cells in the acidic tissues that result from a range of pathological states, including tumours, and that can also translocate cell-impermeable cargo molecules across cell membranes in a pH-dependent manner. The technology is based on the interactions of a water-soluble membrane peptide, which we call pHLIP (pH (Low) Insertion Peptide), with the lipid bilayers of cell membranes. at the normal pH of healthy tissue it binds to cell surfaces, but at low pH pHLIP inserts as a monomer across the cell membrane to form a stable transmembrane helix. pHLIP holds promise for imaging and drug delivery applications.