The objective of this CTMI volume is to provide readers with a foundation for understanding what ADARs are and how they act to affect gene expression and function. Because A-to-I editing may affect base-pairing and RNA structure, processes including translation, splicing, RNA replication, and miR and siRNA silencing may be affected.  It also is becoming increasingly apparent that ADARs may possess roles not only as enzymes that deaminate adenosine to produce inosine in RNA substrates with double-stranded character, but also as proteins independent of their catalytic property. Future studies of ADARs no doubt will provide us with additional surprises and new insights into the modulation of biological processes by the ADAR family of proteins.