Three general effects of mechanical high-frequency excitation (HFE) are described: Stiffening - an apparent change in the stiffness associated with an equilibrium; Biasing - a tendency for a system to move towards a particular state which does not exist or is unstable without HFE; and Smoothening - a tendency for discontinuities to be apparently smeared out by HFE. Studies of specific physical systems as well as more general models are described.