  We produce a highly structured way of associating a simplicial category to a
model category which improves on work of Dwyer and Kan and answers a question
of Hovey. We show that model categories satisfying a certain axiom are Quillen
equivalent to simplicial model categories. A simplicial model category provides
higher order structure such as composable mapping spaces and homotopy colimits.
We also show that certain homotopy invariant functors can be replaced by weakly
equivalent simplicial, or `continuous', functors. This is used to show that if
a simplicial model category structure exists on a model category then it is
unique up to simplicial Quillen equivalence.
