Categorical models for equivariant classifying spaces
  Starting categorically, we give simple and precise models of equivariant classifying spaces. We need these models for work in progress in equivariant infinite loop space theory and equivariant algebraic K-theory, but the models are of independent interest in equivariant bundle theory and especially equivariant covering space theory. 
B. J. Guillou, J. P. May, and M. Merling
math.AT