# SNOMED CT Upgrade

Terminology experts usually maintain their own Code Systems. These can be independent or dependent on international or other terminologies, for example on SNOMED CT, and are called extensions. Mostly, extensions contain modules that are assigned to additional concepts which have their terms written in the maintainer’s native language, or terms for specific usages. Snow Owl supports maintaining multiple levels of dependent extensions and allows isolating them from each other. Moreover, multiple extensions on the same level can also exist simultaneously. For example, it is possible to maintain two SNOMED CT extensions that are based on different versions of the same international SNOMED CT version.

As new versions become available for the underlying extended Code System, it is necessary to be able to upgrade the extension Code System to take advantage of the latest changes. As the upgrade process can introduce changes that break the referential integrity of the extension Code System - for example, inactivated/changed concept in the new version of the extended Code System -, it might be necessary to resolve issues during the process. To perform the upgrade process, check out the links below.

{% content-ref url="/pages/-Mb6TwUh1dcf5xJrrhFH" %}
[Perform Upgrade](/snow-owl-authoring-platform/content-management/snomed-ct-upgrade/perform-upgrade.md)
{% endcontent-ref %}

{% content-ref url="/pages/-Mb6UOiAhEY3I7-OvWhS" %}
[Complete Upgrade Process](/snow-owl-authoring-platform/content-management/snomed-ct-upgrade/complete-upgrade.md)
{% endcontent-ref %}


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