The common autoimmune disease type 1 diabetes provides a paradigm for the genetic analysis of multifactorial disease. Disease occurrence is attributable to the interaction with the environment of alleles at many loci interspersed throughout the genome. Their mapping and identification is difficult because the disease-associated alleles occur almost as commonly in patients as in healthy individuals; even the highest-risk genotypes bestow only modest risks of disease. The identification of common quantitative trait loci (QTL) in autoimmune disease and in other common disorders, therefore, requires a very close marriage of genetics and biology. Two QTLs have been identified in human type 1 diabetes: the major histocompatibility complex HLA class II loci and a promoter polymorphism of the insulin gene. The evidence for their primary roles in disease aetiology demonstrates the necessity of combined studies of genetics and biology. Their functions and interaction underpin an emerging picture of the basic causes of the disease and direct analyses towards other candidate genes and pathways. The genetic tools used for QTL identification include transgenesis and gene knockouts, whole genome scanning for linkage, mouse congenic strains, linkage disequilibrium mapping, and the establishment of ancestral haplotypes among disease-associated chromosomes.