With declarative package management, you specify which packages you want on your system by setting the option environment.systemPackages. For instance, adding the following line to configuration.nix enables the Mozilla Thunderbird email application: environment.systemPackages = [ pkgs.thunderbird ]; The effect of this specification is that the Thunderbird package from Nixpkgs will be built or downloaded as part of the system when you run nixos-rebuild switch. You can get a list of the available packages as follows: $ nix-env -qaP '*' --description nixos.firefox firefox-23.0 Mozilla Firefox - the browser, reloaded ... The first column in the output is the attribute name, such as nixos.thunderbird. (The nixos prefix allows distinguishing between different channels that you might have.) To “uninstall” a package, simply remove it from environment.systemPackages and run nixos-rebuild switch.