Type 1 diabetes (T1D) in children and particularly in teenagers and adults is strongly associated with autoreactivity to the Mr 65,000 isoform of glutamic acid decarboxylase (GAD65). Autoantibodies to GAD65 are common at the time of clinical diagnosis and may be present for years prior to the onset of hyperglycemia. GAD65 autoantibodies predict conversion to insulin dependence when present in patients classified with type 2 diabetes nowadays more often referred to as patients with latent autoimmune diabetes in the adult (LADA) or type 1,5 diabetes. Analyses of T cells with HLA DRB1 0401-tetramers with GAD65-specific peptides as well as of anti-idiotypic GAD65 autoantibodies suggest that GAD65 autoreactivity is common. The immunological balance is disturbed and the appearance of GAD65 autoantibodies represents markers of autoreactive loss of pancreatic beta cells. Extensive experimental animal research, in particular of the Non-obese diabetic (NOD) mouse, showed that GAD65 therapies reduce insulitis and prevent spontaneous diabetes. Recombinant human GAD65 produced by current Good Manufacturing Practice (cGMP) and formulated with alum was found to be safe in Phase I and II placebo-controlled, double-blind, randomized clinical trials. The approach to modulate GAD65 autoreactivity with subcutaneous immunotherapy (SCIT) showed promise as alum-formulated GAD65 induced a dose-dependent reduction in the disappearance rate of endogenous residual C-peptide production. Additional controlled clinical trials are needed to uncover the mechanisms by which subcutaneous injections of recombinant human GAD65 may alter GAD65 autoreactivity.