Foundations and Trends R  in Information Retrieval Vol. 2, Nos. 1–2 (2008) 1–135 c  2008 B. Pang and L. Lee DOI: 10.1561/1500000001 Opinion Mining and Sentiment Analysis Bo Pang 1 and Lillian Lee 2 1 Yahoo! Research, 701 First Avenue, Sunnyvale, CA 94089, USA, bopang@yahoo-inc.com 2 Computer Science Department, Cornell University, Ithaca, NY 14853, USA, llee@cs.cornell.edu Abstract An important part of our information-gathering behavior has always been to find out what other people think. With the growing availability and popularity of opinion-rich resources such as online review sites and personal blogs, new opportunities and challenges arise as people now can, and do, actively use information technologies to seek out and understand the opinions of others. The sudden eruption of activity in the area of opinion mining and sentiment analysis, which deals with the computational treatment of opinion, sentiment, and subjectivity in text, has thus occurred at least in part as a direct response to the surge of interest in new systems that deal directly with opinions as a first-class object. This survey covers techniques and approaches that promise to directly enable opinion-oriented information-seeking systems. Our focus is on methods that seek to address the new challenges raised by sentiment-aware applications, as compared to those that are already present in more traditional fact-based analysis. We include material