  Let S be a site. We introduce the notion of extensions of strictly
commutative Picard S-stacks. We define the pull-back, the push-down, and the
sum of such extensions and we compute their homological interpretation: if P
and Q are two strictly commutative Picard S-stacks, the equivalence classes of
extensions of P by Q are parametrized by the cohomology group Ext^1([P],[Q]),
where [P] and [Q] are the complex associated to P and Q respectively.
