 The idea that underlies naturalness is that the phenomena described by some physical theory should not depend sensitively on specific details of a more fundamental (currently unknown) theory to which it is an effective low-energy approximation. In what follows, the motivation, significance, and implementation of this idea in the framework of quantum field theory are explained. For a more detailed introduction aimed at physicists see Giudice (2008), for one aimed at philosophers of physics see Williams (2015).