With declarative package management, you specify which packages you want on your system by setting the option . For instance, adding the following line to configuration.nix enables the Mozilla Thunderbird email application: = [ 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. Note: the nixos prefix tells us that we want to get the package from the nixos channel and works only in CLI tools. In declarative configuration use pkgs prefix (variable). To “uninstall” a package, simply remove it from and run nixos-rebuild switch.