  Lie algebroids can not always be integrated into Lie groupoids. We introduce
a new object--``Weinstein groupoid'', which is a differentiable stack with
groupoid-like axioms. With it, we have solved the integration problem of Lie
algebroids. It turns out that every Weinstein groupoid has a Lie algebroid and
every Lie algebroid can be integrated into a Weinstein groupoid.
