Photochemical internalization (PCI) is a novel technology for release of endocytosed macromolecules into the cytosol. The technology is based on the use of photosensitizers located in endocytic vesicles. Upon activation by light such photosensitizers induce a release of macromolecules from their compartmentalization in endocytic vesicles. PCI has been shown to increase the biological activity of a large variety of macromolecules and other molecules that do not readily penetrate the plasma membrane, including type I ribosome-inactivating proteins, immunotoxins, plasmids, adenovirus, various oligonucleotides, dendrimer-based delivery of chemotherapeutica and unconjugated chemotherapeutica such as bleomycin and doxorubicin. This review will present the basis for the PCI concept and the most recent significant developments.