  A general semiclassical approach to quantum systems with system-bath
interactions is developed. We study system decoherence in detail using a
coherent state semiclassical wavepacket method which avoids singularity issues
arising in the usual Green's function approach. We discuss the general
conditions under which it is approximately correct to discuss quantum
decoherence in terms of a ``dephasing'' picture and we derive semiclassical
expressions for the phase and phase distribution. Remarkably, an effective
system wavefunction emerges whose norm measures the decoherence and is
equivalent to a density matrix formulation.
