Adeno-associated virus (AAV) based in vivo gene therapy has made great progress in recent years including an approved treatment (Glybera) in Europe and several successful clinical trials in the United States for hemophilia B and Leber congenital amaurosis. One drawback of the vector is the large quantity of virus needed to induce therapeutic levels of transgene expression and thus, production of the vector must be as efficient as possible. Fundamental aspects of AAV’s capsid assembly remain poorly characterized, but the recent discovery of assembly-activating protein (AAP) may be the key to finally understanding this crucial part of recombinant AAV (rAAV) vector production and AAV biology.