(November 30, 2010) Professor Leonard Susskind continues his discussion on T-Duality; explains the theory of D-Branes; models QFT and QCD; and introduces the application of electromagnetism. String theory (with its close relative, M-theory) is the basis for the most ambitious theories of the physical world. It has profoundly influenced our understanding of gravity, cosmology, and particle physics. In this course we will develop the basic theoretical and mathematical ideas, including the string-theoretic origin of gravity, the theory of extra dimensions of space, the connection between strings and black holes, the "landscape" of string theory, and the holographic principle.