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+## TPP
+<small class="subtitle">
+  <a href="https://github.com/opensafely-core/ehrql/blob/main/ehrql/backends/tpp.py">
+    <code>ehrql.backends.tpp.TPPBackend</code>
+  </a>
+</small>
+
+[TPP](https://tpp-uk.com/) are the developers and operators of the
+[SystmOne](https://tpp-uk.com/products/) EHR platform. The ehrQL TPP backend
+provides access to primary care data from SystmOne, plus data linked from other
+sources.
+
+### Patients included in the TPP backend
+
+SystmOne is a primary care clinical information system
+used by roughly one third of GP practices in England,
+with records for approximately 44% of the English population.
+
+Only patients with a full GMS (General Medical Services) registration are included.
+
+We have registration history for:
+
+* all patients currently registered at a TPP practice
+* all patients registered at a TPP practice any time from 1 Jan 2009 onwards:
+    * who have since de-registered
+    * who have since died
+
+A patient can be registered with zero, one, or more than one practices at a given
+time. For instance, students are often registered with a practice at home and a
+practice at university.
+
+!!! warning
+    Refer to the [discussion of selecting populations for studies](../explanation/selecting-populations-for-study.md).
+
+This backend implements the following table schemas:
+
+ * [core](schemas/core.md)
+ * [raw.core](schemas/raw.core.md)
+ * [tpp](schemas/tpp.md)
+ * [raw.tpp](schemas/raw.tpp.md)
+ * [smoketest](schemas/smoketest.md)
+
+## EMIS
+<small class="subtitle">
+  <a href="https://github.com/opensafely-core/ehrql/blob/main/ehrql/backends/emis.py">
+    <code>ehrql.backends.emis.EMISBackend</code>
+  </a>
+</small>
+
+!!! warning
+    Research access to the backend provided by EMIS is temporarily unavailable,
+    pending funding arrangements between NHS England and EMIS.
+    When funding has been secured,
+    we will publish a timeline for gradually reopening access.
+
+[EMIS Health](https://www.emishealth.com/) are the developers and operators of the
+[EMIS Web](https://www.emishealth.com/products/emis-web) EHR platform. The ehrQL
+EMIS backend provides access to primary care data from EMIS Web, plus data linked
+from other sources.
+
+This backend implements the following table schemas:
+
+ * [core](schemas/core.md)
+ * [raw.core](schemas/raw.core.md)
+ * [raw.emis](schemas/raw.emis.md)
+ * [emis](schemas/emis.md)
+ * [smoketest](schemas/smoketest.md)