Our knowledge as to how to solve software engineering problems is increasingly being encapsulated in tools. These tools are at their strongest when they operate in a pre-existing development environment that can provide integration with existing elements such as compilers, debuggers, profilers and visualizers. Some also exist beyond development time and work with the runtime. A further challenge is to develop tools that can span different – and future - development environments and runtimes. This workshop should of interest to all those interested in developing tools as plug-ins for IDEs, runtimes and browsers. We will examine the categories of problems that are best solved in this way, and look at the future challenges.