  We study the phenomenology of gauge singlet extensions of the Standard Model
scalar sector and their implications for the electroweak phase transition. We
determine the conditions on the scalar potential parameters that lead to a
strong first order phase transition as needed to produce the observed baryon
asymmetry of the universe. We analyze the constraints on the potential
parameters derived from Higgs boson searches at LEP and electroweak precision
observables. For models that satisfy these constraints and that produce a
strong first order phase transition, we discuss the prospective signatures in
future Higgs studies at the Large Hadron Collider and a Linear Collider. We
argue that such studies will provide powerful probes of phase transition
dynamics in models with an extended scalar sector.
