  There is strong evidence that new physical degrees of freedom and new
phenomena exist and may be revealed in future collider experiments. The best
hints of what this new physics might be are provided by electroweak symmetry
breaking. I briefly review certain theories for physics beyond the standard
model, including the top-quark seesaw model and universal extra dimensions. A
common feature of these models is the presence of vector-like quarks at the TeV
scale. Then I discuss the role of a linear $e^+e^-$ collider in disentangling
this new physics.
