The quality of knowledge that a system has substantially influences its performance. Often, the terms "knowledge", its "quality" and how it is "measured" or "valuated", are left vague enough to accommodate several ad hoc interpretations. This paper articulates two definitions of knowledge and their associated value measures. The paper focuses on the theory underlying measurements and its application to knowledge valuation; it stresses the issue of constructing meaningful measures rather than discussing some of the desirable properties of measures (e.g. reliability or validity). A detailed example of knowledge valuation using the measures is described. The example demonstrates the importance for system understanding and the difficulty of valuating knowledge. It shows the importance of employing several different measures simultaneously for a single valuation. The paper concludes by discussing the scope of and relationships between the measures.