We are delighted to present the technical program of the 24th International World Wide Web Conference (WWW 2015), held from May 18 to 22, 2015 in Florence, Italy. Since its inception in 1994, the International World Wide Web Conference has been the premier venue for researchers, academics, businesses, and standards organizations to convene and discuss the latest Web research and technologies. This year, we have received a record number of 929 submissions for the research program, distributed over the following 11 Web-related technical areas: (1) Behavior Analysis and Personalization, (2) Crowdsourcing Systems and Social Media, (3) Content Analysis, (4) Internet Economics and Monetization, (5) Pervasive Web and Mobility, (6) Security and Privacy, (7) Semantic Web, (8) Social Networks and Graph Analysis, Web Infrastructure -- Data Centers, (9) Content Delivery Networks, (10) Web Mining, and (11) Web Search Systems and Applications. All submissions underwent a rigorous reviewing process, where at least three expert Program Committee (PC) members reviewed each submission and a fourth expert lead the discussion of the reviews. In total, a highly skilled and diverse team of 554 PC members were involved in the reviewing process and they collectively produced over 3000 reviews. The PC members were recruited by area chairs, who also oversaw the reviewing of the submissions in their area, made initial recommendations to accept or reject the submissions in their area, and participated in a two-day area chair meeting held in Kaiserslautern, Germany. The final acceptance decisions were made at the meeting, where the recommendations of the area chairs were discussed and voted on by all area chairs and the PC chairs. Submissions, where area chairs were conflicted, were handled separately PC chairs. The outcome of the meeting was to accept a total of 131 (14.1%) papers for the research program. We believe that these 131 accepted papers represent some of the best Web-related research conducted over the last year and we are confident that they would lead to fruitful discussions and new ideas at the conference.