In this chapter we briefly described the main methods of reasoning: induction, a type of learning by making generalizations from examples; abduction, or explanation by reasoning back from a situation to the state or action that produced it; spatial reasoning, by forming a mental image; and deductive reasoning using classical logic. One popular method of representing the meaning of words in a way that enables a computer to carry out reasoning is the semantic network. A semantic network describes the relationships between words, by means of directed links. We introduced three special types of link: isa, ako, and connects. The isa link relates an item (Tweetie, or boxB) to its general class (canary, box), and ako links a class (canary, box) to a super-class (bird, container). Both links are transitive -- that is, they allow inferences like ``If Tweetie is a canary and a canary is a kind of bird, then Tweetie is a bird.'' The isa and ako links permit propert inheritance, a form of inferencing whereby properties of a class or super-class are inherited by the subclass or instance. Given that Tweetie is a canary and canaries have the property `yellow', an inference can be made that Tweetie has the property `yellow'. The connects link is both transitive and commutative. If A connects B and B connects C, then we can infer that A connects C, C connects A, B connects A, and C connects B. The meaning of generic nodes (words like `canary' that stand for a class of objects) can be interpreted in different ways, and we introduced two interpretations: sets (`canary' represents the set of individuals like Tweetie and friends, and is a subset of `bird') and prototypes (a `canary' is prototypically yellow and can sing).