The MSR Challenge aims at offering researchers and practitioners in the area of Mining Software Repositories a shared set of software repositories, enabling them to compare their tools and approaches. This year, the main theme of the challenge was on the comparison of projects. We selected four open source projects, and challenged participants to use their brains, tools, computational power, and magic to compare them and uncover interesting similarities and differences. The projects were Eclipse and Netbeans, two popular IDEs written in Java (Group 1) and Firefox and Chrome, two web browsers written in C/C++ (Group 2). We encouraged participants to analyze more than one project, ideally in the same group but allowed them to analyze a single project.