Before we look into how specification of knowledge helps us to solve the validation and verification problem, let us explore why the basic problems of V~zV of expert systems still remain vary challenging. Even though there are different opinions on how V&V should be defined, many researchers within expert sy.stern community accept the definition of V&V as originally defined in software engineering. That is, an expert system is said to be validated if it can be shown that it performs or behaves according to the requirements. On the other hand, an expert system is said to be verified if it can be proven that the 'programming portion of the expert system' meets its specification. Here, by the 'programming portion of an expert system' we mean the set of rules, or frames, or whatever the specific representation used to encode the knowledge. The specification is then a restatement of the requirement in the view of the developer of an expert system.