Although refrigeration systems associated with air-conditioning may be the subject of good maintenance procedures, it is common to find that little or no plant performance information is obtained. This document offers an approach to performance monitoring of medium- to large-sized liquid-chilling systems associated with air conditioning, i.e., those systems of >10kW electrical power input. The reader is guided through each stage of the monitoring process based upon work carried out by BSRIA on several sites. Although cooling systems vary widely in their composition and size, the principles embodied here should enable system owners and operators to tackle monitoring with confidence. The notes assume no previous knowledge of monitoring technology, although they do assume a basic familiarity with refrigeration plant.