  The Baumslag-Solitar groups:
  BS(m,n)=<x,y| x y^{m} x^{-1} = y^{n}> are some of the simplest interesting
infinite groups which are not lattices in Lie groups. They have been studied in
depth from the point of view of combinatorial group theory. It is natural to
ask if the geometric approach to the theory of infinite groups, which has been
so successful in the study of lattices, can yield any insights in this
nonlinear case. We show that in contrast to the solvable groups studied by Farb
and Mosher, the groups BS(m,n) with 1<m<n are all quasi-isometric. This
generalizes to almost all groups acting on trees with infinite cyclic vertex
and edge stabilizers.
