CHL Compactifications and Beyond

This is the transcript of a talk given at the 3rd Simons Workshop in Mathematics and Physics on July 26, 2005. We review the genesis of the CHL (Chaudhuri-Hockney-Lykken) project, explaining both its phenomenological goals and theoretical justification in light of the known vast proliferation of N=1 string vacua. We explain what a CHL compactification is, review some key results such as the construction of moduli spaces with a small number of massless scalar fields, the decompactification of such moduli spaces to one of five consistent ten-dimensional superstring theories, and the appearance of electric-magnetic duality in only the four-dimensional moduli spaces, a 1995 observation due to Chaudhuri & Polchinski.