diff --git a/lib/mongoid/config.rb b/lib/mongoid/config.rb
index <HASH>..<HASH> 100644
--- a/lib/mongoid/config.rb
+++ b/lib/mongoid/config.rb
@@ -280,6 +280,11 @@ module Mongoid #:nodoc
     #
     # @return [ Array<Mongo::DB> ] The slave DB instances.
     def slaves=(dbs)
+      warn(
+        "Using Mongoid for traditional slave databases will be removed in the " +
+        "next release in preference of replica sets. Please change your setup " +
+        "accordingly."
+      )
       return unless dbs
       dbs.each do |db|
         check_database!(db)
Adding in warning for use of slave databases - replica sets should be used from now on.