Knowledge is an important characteristic of human intelligence and reflects the complexity of human languages. To this end, many efforts have been devoted to organizing various human knowledge to improve the ability of machines in language understanding, such as world knowledge, linguistic knowledge, commonsense knowledge, and domain knowledge. Starting from this chapter, our view turns to representing rich human knowledge and using knowledge representations to improve NLP models. In this chapter, taking world knowledge as an example, we present a general framework of organizing and utilizing knowledge, including knowledge representation learning, knowledge-guided NLP, and knowledge acquisition. For linguistic knowledge, commonsense knowledge, and domain knowledge, we will introduce them in detail in subsequent chapters considering their unique knowledge properties.