In the past year, three clinical trials of gene therapy for haemophilia have been initiated. Years of preclinical studies have culminated in translation of research findings into the clinical arena. It is too early to predict which, if any, of these strategies will show efficacy. This paper will review basic aspects of gene therapy for haemophilia and will briefly outline current clinical trials. The three clinical trials all share a dose escalation design. The ongoing trial for haemophilia B involves the intramuscular administration of an adeno-associated virus (AAV) vector expressing human factor IX. In preclinical studies, this strategy has produced therapeutic levels of circulating factor IX in haemophilic mice and dogs.