Adeno-associated virus (AAV) is a member of the parvovirus family, single-stranded DNA-containing nonenveloped icosahedral viruses. AAVs have been regarded as promising vectors for human gene therapy as they have the capacity to establish long-term latency within human cells without any apparent pathogenicity. However, a lot of obvious defects in their applications have been revealed recently, including the paucity of cell surface receptors on some cells, the lack of site-specific integration by recombinant AAV vectors, and the host immune responses to AAV capsid components and transgene products and so forth. Driven by these defects, increasing efforts are being made to study biological properties and infectious pathway of AAVs. It is consequently optimized by the modification of the AAV vectors to produce new generation of recombinant AAV vectors with more security, efficiency and site-specific targeting, allowing AAVs to move forward into broader clinical application.