Developing subsets is an essential part of implementing SNOMED CT®. A SNOMED CT® subset is a constrained set of components for a specific use case. Subsets are used to bind terminology to information models, to constrain the valid values entered on a form, to scope a translation or mapping activity, and to specify rules for clinical decision support and criteria for data retrieval. This chapter explores SNOMED CT® subsets, and explains their purpose, principles, and development processes. It also considers tooling and training that can support the development of SNOMED CT® subsets.