In this chapter, we are concerned with the relationship between Description Logics and other formalisms, regardless of whether they were designed for knowledge representation issues or not. We concentrated on those representation formalisms that either (1) had or have a strong influence on Description Logics (e.g., modal logics), (2) are closely related to Description Logics for historical reasons (e.g., semantic networks and structured inheritance networks), or (3) have similar expressive power (e.g., semantic data models). There are far more knowledge representation formalisms than those mentioned in this section. For example, “verb-centered” graphical formalisms like those introduced by Simmons  are not mentioned since we believe that their relationship with Description Logics is too weak. 4.1 AI knowledge representation formalisms In artificial intelligence (AI), various “non-logical” knowledge representation formalisms were developed, motivated by the belief that classical logic is inadequate for knowledge representation in AI applications. This belief was mainly based upon cognitive experiments carried out with human beings and the wish to have representational formalisms that are close to the representations in human brains. In this Section, we will discuss some of these formalisms, namely semantic networks, frame systems, and conceptual graphs. The first two formalisms are mainly presented for historical reasons since they can be regarded as ancestors of Description Logics. In contrast, the third formalism can be regarded as a “sibling” of Description Logics since both have similar ancestors and live in the same time.