Most industrial refrigeration systems use a multiplicity of compressors to meet loads on each suction pressure level for a given system. Because the loads are variable, the capacity of the operating compressors must vary to match the instantaneous loads to maintain a constant suction pressure. As a compressor unloads, its operating efficiency will be affected. The part-load efficiency characteristics of a compressor depend on a number of factors including: the compression technology, the means for unloading, and the operating compression ratio. This article discusses best practices for compressor sequencing and control.