  A quasi-tree is a geodesic metric space quasi-isometric to a tree. We give a
general construction of many actions of groups on quasi-trees. The groups we
can handle include non-elementary (relatively) hyperbolic groups, rank 1 CAT(0)
groups, mapping class groups and Out(Fn). As an application, we show that
mapping class groups act on finite products of {\delta}-hyperbolic spaces so
that orbit maps are quasi-isometric embeddings. We prove that mapping class
groups have finite asymptotic dimension.
