  The weak units of strict monoidal 1- and 2-categories are already defined. In
this paper, we define them for group-like 1- and 2-stacks. We show that they
form a contractible Picard 1- and 2-stack, respectively. We give their
cohomological description which provides for these stacks a representation by
complexes of sheaves of groups. Later, we extend the discussion to the monoidal
case.
