CSMAR03The definition of a service is keyto the ability to meter it, measure it, regulate it, price it, or otherwise take it into consideration. Reserves (a form of power supply insurance) are key to power system operability. Traditionally, reserves have been characterized in terms of time domain quantities such as ramping rate capabilities and the like. Also, traditionally such measures have not considered the possibility that some of the reserve services may be energy or otherwise time-limited. This paper illustrates how to take into consideration energy-constrained reserve services, and also how to classify and measure both the need for reserves as well as the ability to provide them in terms of frequency domain techniques. In a sense, the use of frequency domain quantities is more natural? For this problem where the characterization of the speed of response is in fact quite important. The paper illustrates a number of numerical examples.