Severe combined immunodeficiencies (SCIDs) correspond to the most severe form of primary immunodeficiency. The extreme severity of the clinical presentation in SCID has legitimately led physicians to consider these conditions as medical emergencies. Hundreds of patients worldwide have undergone allogeneic haematopoietic stem cell transplantation (HCST) in the last 40 years. The complete absence of the T cell compartment in SCID prompted the development (starting in the early 1980s) of haploidentical, parental HSCT for the many patients who do not have a human leucocyte antigen (HLA)-identical sibling. Despite the undeniable progress made in this field over recent years, the long-lasting immunodeficiency that follows partially HLA-incompatible transplantation is still responsible for a mortality rate of 30% at one year post-transplantation. New approaches for reconstituting T cell compartments more rapidly are under intense preclinical development and are discussed herein.