Welcome to the 41st Annual International ACM SIGIR Conference on Research and Development in Information Retrieval (SIGIR 2018), the premier scientific conference in the broad area of information retrieval. We're delighted to host ACM SIGIR 2018 on the beautiful University of Michigan campus in Ann Arbor, U.S.A. We received a high-quality set of submissions for full and short papers, demos, industry papers, tutorials, workshops, and doctoral consortium, to consider for inclusion in the overall conference program. In particular we thank the 15 Track co-Chairs, 93 Senior Program Committee (SPC) members, 343 Program Committee (PC) members, and 91 additional reviewers for their contributions to full and short paper selection. This pool of committed SIGIR volunteers was based in 35 countries/regions and over 255 institutions, spanning academia, industry, and beyond. We reviewed 409 papers for the full paper track, and accepted 86, for an acceptance rate of 21%. In total, there were 1247 authors from 33 countries: 440 from China, 325 from the United States, and 482 from the rest of the world. The top five countries in terms of accepted papers (taking all author affiliations of each paper equally into account) were China (34%), the U.S.A. (30%), Australia (6%), Canada (4%) and Germany (4%). The top three countries in terms of acceptance rates (taking all author affiliations of each paper equally into account) were Australia (44%), Israel (32%) and Germany (31%). The most popular track for submissions was Content Recommendation, Analysis and Classification (124 submissions) while the tracks with highest representation in the accepted papers were Human Factors and Interfaces and Domain-Specific Applications (both 20 papers). This is the first time in recent years that we have adopted a track-based structure in paper submission and review processes. This change was intended to help us attract more submissions from closely related research communities, and the introduction of track chairs (listed below) also helped recruit more domain experts for the program committee and make high quality judgments for the submissions.