Linguistic and commonsense knowledge bases describe knowledge in formal and structural languages. Such knowledge can be easily leveraged in modern natural language processing systems. In this chapter, we introduce one typical kind of linguistic knowledge (sememe knowledge) and a sememe knowledge base named HowNet. In linguistics, sememes are defined as the minimum indivisible units of meaning. We first briefly introduce the basic concepts of sememe and HowNet. Next, we introduce how to model the sememe knowledge using neural networks. Taking a step further, we introduce sememe-guided knowledge applications, including incorporating sememe knowledge into compositionality modeling, language modeling, and recurrent neural networks. Finally, we discuss sememe knowledge acquisition for automatically constructing sememe knowledge bases and representative real-world applications of HowNet.