  The topological structure of field theory often makes inevitable the
existence of stable and unstable localised solutions of the field equations.
These are minima and saddle points of the energy. Saddle point solutions
occurring this way are known as sphalerons, and the most interesting one is in
the electroweak theory of coupled W, Z and Higgs bosons. The topological ideas
underpinning sphalerons are reviewed here.
