Severe combined immunodeficiency (SCID) is the most severe form of inherited primary immunodeficiency and is a paediatric emergency. Delay in recognising and detecting SCID can have fatal consequences and also reduces the chances of a successful haematopoietic stem cell transplant (HSCT). Screening for SCID at birth would prevent children from dying before HSCT can be attempted and would increase the success of HSCT. There is strong evidence to show that SCID fulfills the internationally-established criteria for a condition to be screened for at birth. There is also a test (the T-cell receptor excision circle (TREC) assay) that is now being successfully used in an increasing number of US states to screen for SCID in routine newborn Guthrie samples. Concerted lobbying efforts have highlighted the need for newborn screening (NBS) for SCID, and its implementation is being discussed in Europe both at EU and individual country level, but as yet there is no global mandate to screen for this rare and frequently lethal condition. This paper summarizes the current evidence for, and the success of SCID NBS, together with a review of the practical aspects of SCID testing and the arguments in favour of adding SCID to the conditions screened for at birth.