--- a
+++ b/docs/reference/backends.md
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+Dataset definitions written in ehrQL can be run inside different secure
+environments, managed by different providers of EHR data.
+
+For each such secure environment, there is a corresponding "backend"
+defined in ehrQL. Each ehrQL backend:
+
+* specifies the datasets available inside each secure environment
+* does the necessary translation work to allow the same
+  dataset definition to run against data modelled in different ways and
+  stored in different systems
+
+When writing a dataset definition you don't need to explicitly reference
+any particular backend. But, as not every dataset is available in every
+backend, the [table schema](schemas.md) you use to write your dataset
+definition will determine which backends it can be run against.
+
+Below are the backends currently supported in ehrQL, together with the
+list of [table schemas](schemas.md) each one supports.
+
+
+---8<-- 'includes/generated_docs/backends.md'