Nuclear factor (NF)-kappaB has an important role in immunity and inappropriate NF-kappaB activity has been linked with many autoimmune and inflammatory diseases. Multiple mechanisms normally ensure the proper termination of NF-kappaB activation. In this context, the intracellular ubiquitin-editing protein A20 (also known as Tumor Necrosis Factor Alpha-Induced Protein 3 or TNFAIP3) is a key player in the negative feedback regulation of NF-kappaB signaling in response to multiple stimuli. Moreover, A20 also regulates tumor necrosis factor (TNF)-induced apoptosis. Recent genetic studies demonstrate a clear association between several mutations in the human A20 locus and immunopathologies such as Crohn's disease, rheumatoid arthritis, systemic lupus erythematosus, psoriasis and type 1 diabetes. These findings further illustrate the importance of A20 in the resolution of inflammation and the prevention of human disease.