Applied String Theory

This is a review. Comments are welcome. The observation that the structure of string theory is rich enough to include the standard model in rough outline is an old one, starting with the early constructions of free field constructions, orbifold theories, and in particular Calabi-Yau compactifications in the late 1980s and early 1990s. At the time these constructions provided a large collection of different vacua, with thousands of explicitly constructed Calabi-Yau manifolds, and estimates of vast numbers of bosonic models, each one associated with its own moduli space. It was clear even then that it would be impossible to systematically search this string vacua landscape. This, however, is not a fundamental problem. Adopting the point of view that any physical theory has to describe not only our universe, but all possible consistent universes, leads to the obvious strategy of using some phenomenological input to select viable models among the ocean of models that obviously do not describe physics as we know it.