Protein transduction domains (PTD) or cell penetrating peptides (CTPs) are small peptides able to carry proteins, peptides, nucleic acid, and nanoparticles, including viral particles, across the cellular membranes into cells. In general, PTDs can be classified into 3 types: cationic peptides of 6-12 amino acids in length, comprised predominantly of arginine, ornithine and/or lysine residues; hydrophobic peptides such as leader sequences of secreted growth factors and cytokines; and cell-type specific peptides, identified by screening of peptide phage display libraries. These three types of transduction peptides have many different applications including delivery of therapeutic proteins and drugs, delivery of fluorescent or radioactive compounds for imaging, and improving uptake of DNA, RNA and even viral particles. Here we review the potential applications of protein transduction domains.