This review provides an overview of RNA editing by adenosine deaminases that act on RNA (ADARs), focusing on their biological functions and biochemical mechanisms. ADARs are enzymes that target double-stranded regions of nuclear-encoded RNA and viral RNA, particularly in the nervous system, where they diversify genetic information. They alter codons in mRNAs, create splice sites, and affect RNA structure. ADARs are highly conserved across metazoa and are found in the nucleus, where they bind to dsRNA and catalyze the deamination of adenosines to inosines, which are recognized as guanosines. This review discusses the enzyme family, including ADARs and their related family members, adenosine deaminases that act on tRNAs (ADATs). It highlights the roles of ADARs in regulating protein-protein interactions, mRNA levels, and translational efficiency. The review also explores the phenotypes of animals lacking ADAR activity, the mechanisms of RNA editing in various substrates, and the structures of ADAR substrates. Additionally, it examines the specificity of ADARs, including their preferences and selectivity, and the factors that influence these properties. The review concludes by discussing the catalytic mechanism of ADARs, which involves a tetravalent intermediate and water as the nucleophile.This review provides an overview of RNA editing by adenosine deaminases that act on RNA (ADARs), focusing on their biological functions and biochemical mechanisms. ADARs are enzymes that target double-stranded regions of nuclear-encoded RNA and viral RNA, particularly in the nervous system, where they diversify genetic information. They alter codons in mRNAs, create splice sites, and affect RNA structure. ADARs are highly conserved across metazoa and are found in the nucleus, where they bind to dsRNA and catalyze the deamination of adenosines to inosines, which are recognized as guanosines. This review discusses the enzyme family, including ADARs and their related family members, adenosine deaminases that act on tRNAs (ADATs). It highlights the roles of ADARs in regulating protein-protein interactions, mRNA levels, and translational efficiency. The review also explores the phenotypes of animals lacking ADAR activity, the mechanisms of RNA editing in various substrates, and the structures of ADAR substrates. Additionally, it examines the specificity of ADARs, including their preferences and selectivity, and the factors that influence these properties. The review concludes by discussing the catalytic mechanism of ADARs, which involves a tetravalent intermediate and water as the nucleophile.