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.