  We give a homotopy theoretic characterization of stacks on a site $\cC$ as
the {\it homotopy sheaves} of groupoids on $\cC$. We use this characterization
to construct a model category in which stacks are the fibrant objects. We
compare different definitions of stacks and show that they lead to Quillen
equivalent model categories. In addition, we show that these model structures
are Quillen equivalent to the $S^2$-nullification of Jardine's model structure
on sheaves of simplicial sets on $\cC$.
