Sammendrag: In order to measure, or record, behavior, one has to observe it by way of ones senses, often directly and continously. This is virtually impossible for several main reasons. First, attention is never complete, and features even of simple behaviors will be missing. Second, measuring will always be delayed, and impaired by memory decay. Third, observation is rule governed behavior, and the individual and imperfect nature of language means that there will never be a perfect correspondance between what one tries to observe, or record, and actual observation, or recording. These caveats apply mostly to ongoing behavior. To some extent, a solution is to focus on products of behavior, which can be done more objectively. Practical consequences of these claims are discussed.