This chapter outlines a protocol for the selection by phage display of single-chain variable antibody fragments with dual properties-specificity for tumor cells and the ability to be internalized. The protocol is based on a direct incubation of living target cells with antibody phage display libraries under conditions that allow active endocytosis of phage particles by cancer cells as well as recovery of intracellular phage particles that retain infectivity. This “functional” selection helps avoid the isolation of irrelevant phages that may be obtained when selection is performed on heterogeneous material as a source of antigen. Internalizing antibodies recovered from human antibody repertoires are promising reagents for various therapeutic applications including the delivery of drugs to the cytosol of targeted cell populations.