A web application's attack surface is the combination of URLs it will respond to as well as the inputs to those URLs that can change the behavior of the application. Understanding an application's attack surface is critical to being able to provide sufficient security test coverage, and by watching an application's attack surface change over time security and development teams can help target and optimize testing activities. This presentation looks at methods of calculating web application attack surface and tracking the evolution of attack surface over time. In addition, it looks at metrics and thresholds that can be used to craft policies for integrating different testing activities into Continuous Integration / Continuous Delivery (CI/CD) pipelines for teams integrating security into their DevOps practices.