For many reasons, Nixpkgs cannot accept every package ever created. Things have to change, let's make sure we can at least include every package, even if they are not part of our central repository! Since December 25th 2016, we are now able to use Overlays. Overlays gives us the ability to change the way Nixpkgs see the world, while providing most of the benefits that we can get from it. This presentation will address the following questions: Why overlays are useful, across a community, across an organization, or across the world? How to write an overlay with a few good and bad examples, and digging in the internals of Nixpkgs to understand how overlays are working? What can we do to improve the usage of overlay even more? What parts of Nixpkgs should be converted into overlays?