A model builder typically chooses new quantum fields and their new interactions, attempting to make their combination realistic, testable and physically interesting. In particular, an interesting new model should address questions left unanswered in the Standard Model which has, including three massive neutrinos, 28 free parameters. A model which extends the Standard Model should predict one or more of these parameters or shed light on some other issue such as why there are three quark-lepton families or, the most common motivation, the naturalness or hierarchy problem associated with the quadratic divergences appearing in the scalar sector.