DNA-repair pathways recognise and repair DNA damaged by exogenous and endogenous agents to maintain genomic integrity. Defects in these pathways lead to replication errors, loss or rearrangement of genomic material and eventually cell death or carcinogenesis. The creation of diverse lymphocyte receptors to identify potential pathogens requires breaking and randomly resorting gene segments encoding antigen receptors. Subsequent repair of the gene segments utilises ubiquitous DNA-repair proteins. Individuals with defective repair pathways are found to be immunodeficient and many are radiosensitive. The role of repair proteins in the development of adaptive immunity by VDJ recombination, antibody isotype class switching and affinity maturation by somatic hypermutation has become clearer over the past few years, partly because of identification of the genes involved in human disease. We describe the mechanisms involved in the development of adaptive immunity relating to DNA repair, and the clinical consequences and treatment of the primary immunodeficiency resulting from such defects.