  Generalizing a definition of homotopy fiber products of model categories, we
give a definition of the homotopy limit of a diagram of left Quillen functors
between model categories. As has been previously shown for homotopy fiber
products, we prove that such a homotopy limit does in fact correspond to the
usual homotopy limit, when we work in a more general model for homotopy
theories in which they can be regarded as objects of a model category.
