Opinions in social media are increasingly used by individuals and organizations for making purchase decisions and for marketing and product design. Positive opinions can result in significant financial gains and fames for businesses and individuals. This, unfortunately, gives strong incentives for people to game the system by posting fake positive opinions/reviews to some entities (e.g. products and services) in order to promote them, and/or malicious negative reviews to damage their reputations. Such imposters are called opinion spammers and their activities are called opinion spamming. Fake reviews are rampant on the Internet and are seriously undermining the credibility and trustworthiness of online opinions. Fake reviews come from many different sources. Businesses may write for themselves and also pay individuals, middlemen, and also the so-called “reputation management” firms to write on their behalf. They may also ask their customers to write by giving the customers discounts. As more and more individuals and organizations are using reviews for their decision making, detecting fake reviews has become a pressing issue. Many high profile fake review cases have been reported in the news. I have compiled several press articles and interesting links and listed them in my research page (http://www.cs.uic.edu/~liub/FBS/fake-reviews.html). In this talk, I will first introduce the problem and discuss its major challenges. I will then describe some of our recent work on the topic.