APOBEC3G has been identified as an anti-human immunodeficiency virus type 1 (HIV-1) host factor that belongs to the APOBEC superfamily of cytidine deaminases. It deaminates cytidine to uridine in nascent minus-strand viral DNA, inducing G-to-A hypermutation in the plus-strand DNA of HIV-1. The accumulating evidence demonstrates that APOBEC family proteins also have an antiviral activity against a wide variety of viruses, including not only retroviruses but also other types of viruses, and that each virus seems to have its own strategy for escaping from APOBEC proteins. These results suggest that the APOBEC3 family plays an important role in antiviral host defense as an innate immunity. Recent progress in research on APOBEC family proteins is reviewed.