This paper is based on the idea that, if Information Warfare is important, it must be exercised. If we conduct an IW exercise, we need to perform a quantitative evaluation to learn how to do things better. We can hardly do this if we have no operational (quantifiable) definition of Information, itself. The meaning of “Information” presented in this paper is based on the Shannon definitions of the 1940’s which were the basis for what has become the mathematical theory of information. Situation Assessment is defined as “Systematic generation and use of Information to reduce uncertainty about the composition, readiness, location and status of adversary forces and systems.” The paper contains an explanation of the meanings of, and relationships between messages from an environment; uncertainty; knowledge; information; and situation assessment. It further explains how Information is generated from Messages in the presence of Knowledge, how it is measured and how it is used efficiently to perform Situation Assessment. A working tool for the measurement of information and the design and/or evaluation of situation assessment strategies is described and its use is illustrated with a numerical example. The tool guarantees a very efficient situation assessment strategy and provides performance numbers in the form of throughput, response time and amount of information required to complete the assessment with high confidence.