Recombinant adeno-associated virus (rAAV) is one of the most promising delivery vectors for gene therapy, due to its nonpathogenic property, nonimmunogenecity to host, and broad cell and tissue tropisms. This article summarizes the biological characteristics of AAV; the procedures to prepare, purify, and characterize the rAAV for gene therapy applications; and some of the clinical trials utilizing rAAV as delivery vehicles. Also discussed are the current efforts to modify rAAV to change its tropism, the application of different promoters to accommodate specific transgene expression, and the strategy to expand its capacity.