  We study renormalizable extensions of the standard model that contain
vector-like fermions in a (single) complex representation of the standard model
gauge group. There are 11 models where the vector-like fermions Yukawa couple
to the standard model fermions via the Higgs field. These models do not
introduce additional fine- tunings. They can lead to, and are constrained by, a
number of different flavor- changing processes involving leptons and quarks, as
well as direct searches. An interesting feature of the models with strongly
interacting vector-like fermions is that constraints from neutral meson mixings
(apart from CP violation in neutral kaon mixing) are not sensitive to higher
scales than other flavor-changing neutral-current processes. We identify order
1/(4 pi M)^2 (where M is the vector-like fermion mass) one-loop contributions
to the coefficients of the four-quark operators for meson mixing, that are not
suppressed by standard model quark masses and/or mixing angles.
