Fuzzy sets are a generalization of the well-known concept of sets. They extend sets by allowing elements to partially belong to a set. Each element is assigned a membership degree from the [0,1] interval that tells “how much” the element belongs to a given set. Fuzzy sets capture uncertainty in human languages, which contain vague expressions such as good grades, low pollution, and cold weather. They are a useful tool for solving many natural language processing problems. As such, they play a critical role in the social media data analysis framework presented later in this book. This chapter focuses on explaining the basic ideas behind fuzzy sets and concepts related to the proposed social media analysis framework.