Equivalent System Mass (ESM) is the basis of the Advanced Life Support Research and Technology Development metric for measurement of progress of the Advanced Life Support (ALS) Project under the Advanced Human Support Technology (AHST) Program. ESM may be used to evaluate a system or technology based upon its mass, volume, power, cooling and manpower requirements. The ESM metric as defined in the ALS Research and Technology Development Metric Baseline is International Space Station (ISS) technology ESM divided by the ALS technology ESM for a specified mission.This paper discusses various theoretical and practical issues behind application of ESM to systems as well as to individual technologies. Difficulties that might be encountered by researchers in application of the metric are addressed.It is crucial that ALS researchers be proficient in assessing technologies and/or systems of interest with ESM, to minimize the chance of misapplication of the approach. In this paper, a technique for determining the ESM of a subsystem is explained, and an example subsystem application of ESM is performed.