Welcome to MSR 2018, the Fifteenth International Conference on Mining Software Repositories, held on May 28 and 29 in Gothenburg, Sweden, co-located with the 40th International Conference on Software Engineering (ICSE 2018). In 2004 the first highly successful International Workshop on Mining Software Repositories was held in Edinburgh, Scotland. In the 14 years since, MSR has grown into a thriving community that organises a large, mature and reputable yearly conference. This year we had more than 200 submissions in the main track categorised as research papers, reusable tools, and practical experiences, along with the Mining Challenge and Data Showcase tracks. Based on feedback from within the community, MSR used double-blind reviewing for the second year in a row to reduce reviewer bias as much as possible and ensure fairness in the review process.