There are a number of specialties in low-dimensional topology that can
find in their family tree a common ancestry in the theory of surface
mappings. These include knot theory as studied through the use of braid
representations, and 3-manifolds as studied through the use of Heegaard
splittings. The study of the surface mapping class group (the modular
group) is of course a rich subject in its own right, with relations to
many different fields of mathematics and theoretical physics. However,
its most direct and remarkable manifestation is probably in the vast
area of low-dimensional topology.