  We find canonical decompositions for finitely presented groups which
specialize to the classical JSJ-decomposition when restricted to the
fundamental groups of Haken manifolds. The decompositions that we obtain are
invariant under automorphisms of the group. A crucial new ingredient is the
concept of a regular neighbourhood for a finite family of almost invariant
subsets of a group. An almost invariant set is an analogue of an immersion.
