  Let S be a site. We introduce the 2-category of biextensions of strictly
commutative Picard S-stacks. We define the pull-back, the push-down, and the
sum of such biextensions and we compute their homological interpretation: if
P,Q and G are strictly commutative Picard S-stacks, the equivalence classes of
biextensions of (P,Q) by G are parametrized by the cohomology group Ext^1([P]
{\otimes} [Q] ,[G]), the isomorphism classes of arrows from such a biextension
to itself are parametrized by the cohomology group Ext^0([P]{\otimes} [Q] ,[G])
and the automorphisms of an arrow from such a biextension to itself are
parametrized by the cohomology group Ext^{-1}([P]{\otimes}[Q] ,[G]), where
[P],[Q] and [G] are the complex associated to P,Q and G respectively.
