SNOMED CT was a joint development between the NHS in England and the College of American Pathologists (CAP). It was formed in 1999 by the convergence of SNOMED RT and the United Kingdom's Clinical Terms Version 3 (formerly known as the Read Codes).  

SNOMED itself was started in 1965 as SNOP (Systematized Nomenclature of Pathology), and later extended into other medical fields.

In 2007, the SNOMED CT intellectual property rights were transferred from the CAP to the SNOMED SDO® in the formal creation of the IHTSDO.

At the time that IHTSDO purchased the rights to SNOMED CT from the College of American Pathologists (CAP), IHTSDO also purchased rights to "antecedent intellectual property" owned by CAP, including SNOP, SNOMED, SNOMED II, SNOMED International (3.x), and SNOMED RT. Some of these old versions (or local variations or language translations which have developed over time) are still in use, particularly for histology results reporting. Many of these may no longer be officially licensed and exist only in paper format held by clinicians that are not maintained over time. 

At its meeting of October 2009 the highest authority of the IHTSDO, the General Assembly made the following decisions, which should be communicated via a public announcement that:

1. The IHTSDO is not maintaining antecedent (old) versions of SNOMED, such as SNOP, SNOMED, SNOMED II, SNOMED International (3.x), and SNOMED RT. Since both clinical science and the terminology itself have evolved considerably since they were developed, use of these older versions may involve risk. IHTSDO cannot be responsible for this risk.
2. The announcement is issued jointly by the IHTSDO and its Members to strongly encourage users of antecedent versions to put appropriate plans in place as soon as possible to migrate to the current version of SNOMED CT. Some resources exist that may help in this process, such as bridging files for SNOMED II and SNOMED 3.5 to SNOMED RT.
3. In order to maintain continuity for existing users of antecedent versions, it is proposed that they will be able to continue licensing these products until 26th April 2017. IHTSDO and its Members propose to stop issuing (directly or indirectly) licenses for the use of antecedent versions including SNOP, SNOMED, SNOMED II, SNOMED International (3.x), and SNOMED RT as of April 26th 2017 except for research purposes and to enable the interpretation of historical data captured using antecedent versions.
4. Individual Members of IHTSDO may, as is their right, institute policies that antecedent versions of SNOMED should not be used within their jurisdictions prior to 26th April 2017.
5. This announcement has the clinical support of the World Association of Societies of Pathology and Laboratory Medicine through the International Pathology and Laboratory Medicine Special Interest Group of the IHTSDO

Any current user of an antecedent version of SNOMED, who wishes for further information or help, should contact the IHTSDO. Please note that the International Release of SNOMED CT includes migration tables between antecedent versions that may be of assistance to those who are preparing to move from antecedent versions of the terminology to SNOMED CT. For more information, please see the Technical Reference Guide. Furthermore, a number of additional maps are also available from country National Release Centres. Maps available internationally include a SNOMED II to SNOMED RT bridge file and a SNOMED 3.5 to SNOMED RT bridge file. More