In this chapter, phage-display peptide technology has been used to select peptides that internalize into breast-cancer cells. The used biopanning procedure provides information on the best peptide to be used. When one of the selected peptides was conjugated to an antisense oligonucleotide against the ErbB2 receptor, specific delivery to breast-cancer cells was demonstrated. The established biopanning procedure should help in the rational selection of cancer-targeting peptides for specific delivery of DNA and RNA oligonucleotides into cancer cells.