This is an expository article invited for the ``Commentary'' section of PNAS
in connection with Y.-Z. Huang's article, ``Vertex operator algebras, the
Verlinde conjecture, and modular tensor categories,'' appearing in the same
issue of PNAS. Huang's solution of the mathematical problem of constructing
modular tensor categories from the representation theory of vertex operator
algebras is very briefly discussed, along with background material. The
hypotheses of the theorems entering into the solution are very general, natural
and purely algebraic, and have been verified in a wide range of familiar
examples, while the theory itself is heavily analytic and geometric as well as
algebraic.