In modern world, Sentiment Analysis or opinion mining is the computational study of people opinions, sentiments, attitudes, and emotions expressed in written language. It is one of the most active research areas in natural language processing and text mining. Its popularity is mainly due to two reasons. First, it has a wide range of applications because opinions are central to almost all human activities and are key influencer of our behaviors. Whenever we need to make decision, we want to hear others’ opinions. Second, it presents many challenging research problems, which had never been attempted before the year 2000.