The Ninth International Working Conference on Mining Software Repositories was held June 2-3 in Zurich, Switzerland, and co-located with the 2012 International Conference on Software Engineering (ICSE 2012). The first decade of MSR is coming quickly to and end: This is the fifth year that MSR has been a Working Conference, building on the strong foundation of four years as a successful ICSE workshop. We are happy to report that the community of MSR researchers continues to grow, while at the same time the research being performed is maturing. This year saw a further increase in the total number of submissions: 64 full paper submissions (10 pages) and 22 short paper submissions (4 pages) for a total of 86. This total is 8 more than the 78 total submissions in 2011. Full paper submissions increased by 3 (64 vs. 61) while short paper submissions increased by 5 (22 vs. 17). Of the 64 full paper submissions, 18 were ultimately accepted, for a full paper acceptance rate of 28%. For short papers, of the 22 submissions 11 were accepted, for a short paper acceptance rate of 50%. Following last year's conference, MSR 2012 accepted or rejected papers at their submitted length, and did not permit downgrading of a paper from full paper to short paper. Each paper received three reviews by members of the program committee. After we received all reviews, an online discussion process allowed variance among reviews to be understood, and in many cases, consensus to emerge. The program committee co-chairs then personally read all of the reviews and commentary prior to making final decisions. In the content of the papers, this year sees a continuation of previous topics, as well as the emergence of some new ones, such as the mining of mobile application repositories. Following MSR 2011, in MSR 2012, each full paper receives a 15-minute presentation slot and each short paper receives a 7-minute presentation slot. In keeping with MSR's roots as a working conference, each session includes significant time for discussion. The conference has two keynote presentations this year.