Type 1 diabetes (T1D) is a chronic autoimmune disease characterized by selective destruction of pancreatic ${eta}$-cells resulting in insulin deficiency. The genetic determinants of T1D susceptibility have been linked to several loci, in particular to the human leukocyte antigen (HLA) region, which accounts for 50% of the genetic risk of developing T1D. Multiple genes in the HLA region, which are in strong linkage disequilibrium, are thought to be involved. Another important locus, with a smaller effect on genetic predisposition to T1D, is the insulin gene. The advent of numerous single nucleotide polymorphism markers and genome screening has enabled the identification of dozens of new T1D susceptibility loci. Some of them appear to predispose to T1D independently of the HLA and may be important in families with T1D who lack strong HLA susceptibility. Other loci may interact with each other to cause susceptibility. The autoimmune response against ${eta}$-cells can also be triggered by environmental factors in the presence of a predisposing genetic background. Deciphering the environmental and genetic factors involved should help to understand the origin of T1D and aid in the design of individualized prevention programs.