Numerous gene therapy vectors, both viral and non-viral, are taken into the cell by endocytosis, and for efficient gene delivery the therapeutic genes carried by such vectors have to escape from endocytic vesicles so that the genes can further be translocated to the nucleus. Since endosomal escape is often an inefficient process, release of the transgene from endosomes represents one of the most important barriers for gene transfer by many such vectors. To improve endosomal escape we have developed a new technology, named photochemical internalisation (PCI). In this technology photochemical reactions are initiated by photosensitising compounds localised in endocytic vesicles, inducing rupture of these vesicles upon light exposure. The technology constitutes an efficient light-inducible gene transfer method in vitro, where light-induced increases in transfection or viral transduction of more than 100 and 30 times can be observed, respectively. The method can potentially be developed into a sitespecific method for gene delivery in vivo. This article will review the background for the PCI technology, and several aspects of PCI induced gene delivery with synthetic and viral vectors will be discussed. Among these are: (i) The efficiency of the technology with different gene therapy vectors, (ii) use of PCI with targeted vectors, (iii) the timing of DNA delivery relative to the photochemical treatment. The prospects of using the technology for site-specific gene delivery in vivo will be thoroughly discussed, with special emphasis on the possibilities for clinical use. In this context our in vivo experience with the PCI technology as well as the clinical experience with photodynamic therapy will be treated, as this is highly relevant for the clinical use of PCI-mediated gene delivery. The use of photochemical treatments as a tool for understanding the more general mechanisms of transfection will also be discussed.