Web application server (WAS), which is considered to be one of the most exciting milestones of enterprise software technology since the relational database and has become very popular in the last few years. It solves the problems in applying traditional middlewares to web computing environment and provides web middleware platforms that support for the deployment, integration, and execution of transactional web applications. Attention to the new category of middleware has been drawn from the academia and software industry. The current state-of-the-art of Web application servers, including what is a web application server and its functionality, architectural model, component container, transaction processing, load balancing, mid-tier caching, web services and analysis criteria etc., are outlined. Using the Ecperf benchmark, a comparative study among some leading Web application servers is presented by evaluating their functions and performance. Lastly, some open issues about WASs are discussed, and the future direction of WASs is pointed out.