String models Calabi-Yau compactification of heterotic string theory Orbifold compactification of heterotic string theory Grand Unified Theory(GUT) can be realized naturally through the elegant Es breaking chain: Es D E6 D SO(10)D SU(5) D-brane models on Type Il orientifolds N stacks of D-branes gives us U(N) gauge symmetry: Pati-Salam Models Free fermionic string model building Realistic models with clean particle spectra can only be constructed t the Kac-Moody level one: the Standard-like models, Pati-Salam mode els, and flipped SU(5) models