The invention provides a herpes simplex virus (HSV) vector that does not express toxic HSV genes in non-complementing cells and which comprises a genome comprising one or more transgenes, wherein the vector is capable of expression of a transgene for at least 28 days in non-complementing cells. The disclosed vectors include vectors having deletions in the genes ICP0, ICP4, TCP22, TCP27 and TCP47, or alternative inactivating mutations, or vectors which express one or more of these genes with modified kinetics. The invention also relates to viral stocks of the inventive vectors, compositions thereof suitable for use therapeutically or for in vitro applications, and methods relating thereto. In another aspect, the invention provides a complementing cell, in particular a U20S cell, engineered to express ICP4 and ICP27 when the cell is infected with HSV for the production of the inventive vector. Said cells are disclosed as naturally complementing ICP0.