Capital utilisation and the physical stock of capital are generally assumed to be close substitutes in the production process. Thus, models of the demand for capital or of investment activity must, in principle, incorporate some notion of the optimal level of capital utilisation and its associated costs. The part played by capital utilisation in investment activity is the primary reason for this paper and it is argued that the literature in this area has tended to underemphasize its role. The discussion, however, focusses on the reasons for the lack of emphasis on capital utilisation and this is traced to both problems of theory and measurement. A simple neoclassical theoretical framework is adopted throughout, although it is fairly clear that a number of further problems will be posed by the introduction of capital utilisation into a vintage model. While the theoretical discussion is perfectly general, the empirical work uses data for the U.K. Chemicals industry, simply in order to illustrate the problems of measurement.