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