Understanding short texts is crucial to many applications, but challenges abound. Here we focus on short texts which refer to texts with limited context. These short texts are produced, including Search queries, Tags, Keywords, Conversation or Social posts and containing limited context. Semantic knowledge is required in order to better understand short texts. First, short texts do not always observe the syntax of a written language. As a result, traditional natural language processing tools, ranging from part-of-speech tagging to dependency parsing, cannot be easily applied. Second, short texts usually do not contain sufficient statistical signals to support many state-of-the-art approaches for text mining such as topic modeling. Third, short texts are more ambiguous and noisier and are generated in an enormous volume, which further increases the difficulty to handle them. the book deals the knowledge-intensive approaches for discovering semantics of short texts.