There has been much confusion in the NHS in the UK as to the role of, and relationship between, the Read Codes, the ICD and OPCS-4, and HRGs. This paper describes the processes of "terming" (using the terms of the Read Codes), encoding (using the ICD and OPCS-), and grouping (using HRGs). The paper will illustrate how the three processes described are complementary and how all are needed in the chain of converting detailed clinical data from the terms recorded in the patient record into classification assignments required, for example, for national returns and into the groupings required, for example, for contracting in the NHS.