Activation-induced deaminase (AID), a member of the AID/apolipoprotein B mRNA-editing enzyme-catalytic (APOBEC) family, deaminates DNA cytidines into uridines and is the major trans-acting player of immunoglobulin (Ig) genes' diversification in mature B lymphocytes. It allows multiple antigen-driven Ig modifications through gene conversion and/or somatic hypermutation of variable region genes and also permits to switch from IgM expression to other antibody classes after class switch recombination, or to stop Ig expression after locus suicide recombination. AID is expressed at high levels into germinal center activated B cells with a very stringent temporal and spatial regulation. Despite multiple levels of regulation, off-target effects of AID are quite frequent in the B cell lineage and can affect a number of non-Ig genes, albeit at lower level than Ig genes. Beyond the immune system, AID also contributes to cytosine demethylation in undifferentiated cells by deaminating methylcytosines into thymines which are further processed by thymidine glycosylase. This contributes to the maintenance of pluripotency and to the limitation of genetic imprinting. Since AID attacks on DNA can induce replication errors, base excision repair, or mismatch repair, they are strongly mutagenic and are also driving forces of tumorigenesis, not only in B cell malignancies but also in some non-lymphoid tumors involving ectopic AID expression.