Professor Penner's early research interests were in the fields of low-dimensional topology and dynamical systems. His doctoral thesis solved an old problem of Max Dehn and led to a monograph describing the basic theory of train tracks in surfaces. Turning more towards geometry, Professor Penner discovered and developed the decorated Teichmueller theory of punctured surfaces in a series of papers. He has subsequently applied these techinques to various problems both in mathematics and in the string theory of high-energy physics. His current research interests include further development of these interfaces between geometry and physics especially involving new connections with number theory.