diff --git a/README.md b/README.md
index 0b16a24..1f2c8f1 100644
--- a/README.md
+++ b/README.md
@@ -118,3 +118,6 @@ If you find any **bugs** or have a **problem** while using this library, please
 
 ### On Hex.pm
 Worker Pool is available on [Hex.pm](https://hex.pm/packages/worker_pool).
+
+### Requirements
+**Required OTP version 23** or or higher. We only provide guarantees that the system runs on `OTP23+` since that's what we're testing it in, but the `minimum_otp_vsn` is `"21"` because some systems where **worker_pool** is integrated do require it.
diff --git a/rebar.config b/rebar.config
index 09f6400..c23dfda 100644
--- a/rebar.config
+++ b/rebar.config
@@ -2,7 +2,7 @@
 %% ex: ts=4 sw=4 ft=erlang et
 
 %% == Erlang Compiler ==
-{minimum_otp_vsn, "23"}.
+{minimum_otp_vsn, "21"}.
 
 %% Erlang compiler options
 {erl_opts,