Protein transduction domains (PTDs) are small cationic peptides that can facilitate the uptake of large, biologically active molecules into mammalian cells. Recent reports have suggested that PTDs may be able to mediate the delivery of cargo to tissues throughout a living organism. Such technology could eliminate the size restrictions on usable drugs, enabling previously unavailable large molecules to modulate in vivo biology and alleviate disease. In this article, we review the evidence that PTDs can be used both to deliver active molecules to pathological tissue in vivo and to treat models of disease such as ischemia, inflammation, and cancer.