Gene transfer early in development for the treatment of monogenetic and other diseases could overcome major obstacles of intervention in the mature individual. Early gene transfer may prevent the onset of irreversible pathological changes, predispose the individual to immunological tolerance to the introduced protein, take advantage of the high vector to cell ratio, and provide unique access to stem cell/progenitor compartments. The past few years have witnessed the publication of five studies showing long-term correction of monogenetic disorders by fetal gene transfer. Many others have examined the use of new vector systems with therapeutic transgenes, tested their potential for treating diseases in a wide range of organs (including the brain, lung and skin), and examined the hazards of fetal application. This review gives a comprehensive summary of the development of fetal gene transfer over the past few years.