The high energy theory group at Penn studies the fundamental forces of nature, early universe cosmology and mathematical physics. Its activities span the range of these fields: from model building, formal field theory and string theory, to new paradigms for cosmology and the interface of string theory with mathematics. Thus, the Penn theory group is one of the few institutions in the world where extensive development of the formal aspects of string theory has been united with a focus on real phenomena in particle physics, cosmology and gravity. The faculty in the group at present are Vijay Balasubramanian, Mirjam Cvetic, Justin Khoury, Burt Ovrut and Mark Trodden. The group is an integral part of the Center for Particle Cosmology and runs an active Visitor Program.