Describes topologically stable patterns such as vortices,
disinclinations, dislocations, and domain walls in ordered media
(superfluids, liquid and solid crystals, magnets). It also introduces
the basic notions of homotopic group theory and the necessary
algebraic topology constructions. The author is one of the inventors
of topological classification of defects in a theory of ordered media
and this classic paper provides an excellent reference text for
postgraduates and researchers.