In this paper we describe two approaches
to the well-posedness of Lagrangian flows
of Sobolev vector fields.
One is the theory of renormalized solutions
which was introduced by DiPerna and Lions
in the eighties. In this framework
the well-posedness of the flow is a corollary
of an analogous result for the corresponding
transport equation. The second approach
has been recently introduced by Gianluca Crippa
and the author and it is instead based
on suitable estimates performed directly on the
lagrangian formulation.