diff --git a/docs/psa_switch-dev.md b/docs/psa_switch-dev.md
new file mode 100644
index 000000000..3ce1ae614
--- /dev/null
+++ b/docs/psa_switch-dev.md
@@ -0,0 +1,82 @@
+# Implementing Externs in PSA Switch
+Externs in PSA Switch are different than those in Simple Switch. 
+Simple Switch uses externs baked into the BMV2 model where as PSA Switch
+uses externs specific to its own architecture. To understand what this
+translates to in code, look at the counter used in 
+[Simple Switch](https://github.com/p4lang/behavioral-model/tree/master/src/bm_sim)
+and the one in 
+[PSA Switch](https://github.com/p4lang/behavioral-model/tree/master/targets/psa_switch/externs).
+The Simple Switch counter lives inside of `src/bm_sim` where as the 
+counter for PSA Switch lives inside of the target specific directory.
+This doc is meant to provide guidance for developers on how to develop
+new externs in PSA Switch.
+
+## Background Knowledge
+For those who are familiar with the internals of BMV2 feel free
+to skip this section. For those who are new/less familiar with how BMV2
+operates, consider reading `P4Objects.cpp` as a starting point
+as this is where externs will be created. From here you can also see
+what the expected JSON for externs should be. Next, read the changes
+in the following [commit](https://github.com/p4lang/behavioral-model/pull/767).
+It details all the changes made to support the counter extern in PSA.
+It will also be useful to understand on a high level how P4 Runtime
+interacts with PSA Switch. Briefly skimming `runtime_CLI.py` will do.
+
+## High-Level Process
+The following process is meant to serve as a general guide on how
+to develop new externs in PSA Switch.
+
+1. Provide support for emitting the extern in p4c. Currently, only
+   counters declared in P4 are translated to JSON in the p4c for PSA
+   Switch. 
+   This [commit](https://github.com/p4lang/p4c/commit/6d97bcf42f034ca113fa7a654fa998a7e10cba17)
+   is an example for how to emit an extern object to JSON, and
+   this [commit](https://github.com/p4lang/p4c/commit/bd6f231f7e6f24164f5d5156e0fad7a0680f2fa2)
+   is an example for how to emit extern methods to JSON.
+   If the examples aren't clear, one can always refer back to the JSON
+   spec defined 
+   [here](https://github.com/p4lang/behavioral-model/blob/master/docs/JSON_format.md).
+   As for internals of how to the compiler works, check out its docs
+   [here](https://github.com/p4lang/p4c/tree/master/docs).
+2. Create the extern type in PSA Switch. There are several important
+   things to note.
+   1. The extern type must sub-class `bm::ExternType`.
+   2. The extern should live within the `bm::psa` namespace.
+   3. You must register the newly created extern type to the
+      extern factory used by BMV2 to create externs from JSON 
+      using `BM_REGISTER_EXTERN_W_NAME`.
+   4. You must also register any extern methods used by P4 using 
+      `BM_REGISTER_EXTERN_W_NAME_METHOD`.
+   5. Create an `import_yourExternNameHere` dummy method and call 
+      it within `psa_switch.cpp`.
+3. Provide P4 Runtime support if necessary. This means overriding 
+   parts of the PSA Switch/P4 Runtime interface as necessary so 
+   that P4 Runtime can configure/monitor the newly developed extern.
+
+
+Refer to [this](https://github.com/p4lang/behavioral-model/pull/767) 
+as a complete example.
+
+## Common Issues
+Here is a list of all the bugs encountered during the development of
+counters and might serve as a helpful resource for others.
+* The extern can't be created.
+  * Most likely a mis-match between the supplied and expected JSON format.
+  * Did you remember to register the extern into the extern factory?
+* The extern method is not being called inside of an action.
+  * Did you remember to register the extern method? These must be done
+    by the programmer.
+* P4 Runtime can't interact with my extern.
+  * Did you remember to override necessary methods in the P4 Runtime
+    interface?
+* I created a new extern, added it to the Makefile, but it isn't
+  being linked.
+  * Dont forget step 2.5 from above.
+* Everything in BMV2 looks good, but it seems that P4c is
+  failing.
+  * Supporting additional externs might have allowed previous tests
+    marked as XFAIL to pass which will cause P4c to fail. Double
+    check the test cases that have failed and see if this was the
+    issue.
+
+