There are many striking phenomena which are attributed to \quantum coherence". It is natural to wonder if there are new quantum coherence eects waiting to be discovered which could lead to interesting results and perhaps even practical applications. A useful starting point for such discussions is a denition of \quantum coherence". In this article I give a denition of quantum coherence and use a number of illustrations to explore the implications of this denition. I point to topics of current i

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... terest in the elds of cosmology and quantum computation where questions of quantum coherence arise, and I emphasize the impact that interactions with the environment can have on quantum coherence.