  We define a category whose objects are finite etale coverings of an algebraic
stack and prove that it is a Galois category and that it allows one to compute
the fundamental group of the stack.
  We then prove a Van Kampen theorem for algebraic stacks whose simplest form
reads: Let U and V be open substacks of an algebraic stack X with X = U \union
V, let P be a set of base points, at least one in each connected component of
X, U, V and U \inter V, then pi_1(X,P) is the amalgamated sum of pi_1(U,P) and
pi_1(V,P) over pi_1(U \inter V, P).
