Web applications are ubiquitous nowadays. Consequently, the field of Web application security is of ever rising significance. This Dagstuhl seminar was conducted to assemble researchers active in the domain to gain a first comprehensive overview of this young discipline in security research. From a content perspective, the topic was explored in a great variety of directions, including for instance Web browser-based security measures, language-based techniques, software engineering centric methods, run-time enforcement, static analysis, or formal approaches.