  We discuss the operational definition of decoherence in various solid state
systems. In particular, we review, in the context of spin-based solid state
quantum computation, the introduction of $T_1$ and $T_2$ to describe
decoherence in a two level system. We provide a perspective on recent
experiments involving the manipulation of spin coherence in semiconductors, and
discuss specific decoherence and dephasing issues in electron spin-based
quantum dot quantum computer architectures.
