By now, it should be quite clear that words and phrases that convey positive or negative sentiment are instrumental for sentiment analysis. This chapter discusses how to compile such word lists. In the research literature, sentiment words are also called opinion words, polar words, or opinion-bearing words. Positive sentiment words such as beautiful, wonderful, and amazing are used to express some desired states or qualities, while negative sentiment words such as bad, awful, and poor are used to express some undesired states or qualities. In addition to individual words, there are sentiment phrases and idioms – for example, cost an arm and a leg. Collectively, they are called the sentiment lexicon (or opinion lexicon). From now on, when we say sentiment words, we mean both individual words and phrases.