Arroyo is an open source, cross-platform C++ class library project designed for modeling of electromagnetic wave propagation through atmospheric turbulence and adaptive optics systems. This paper describes the functionality available in the library and discusses future plans for this project. ; © 2004 Society of Photo-Optical Instrumentation Engineers (SPIE). The author acknowledges the contributions of the many colleagues who have offered advice and support in the development of Arroyo. Richard Dekany, Mitchell Troy, Don Gavel and Jose Milovich deserve particular mention. The author thanks Alan Morrissett for contributing to the assembly of this project into a formal release. The Thirty Meter Telescope (TMT) Project is a partnership of the Association of Universities for Research in Astronomy (AURA), the Association of Canadian Universities for Research in Astronomy (ACURA), the California Institute of Technology and the University of California. The partners gratefully acknowledge the support of the Gordon and Betty Moore Foundation, the US National Science Foundation, the National Research Council of Canada, the Natural Sciences and Engineering Research Council of Canada, and the Gemini Partnership. ; Published - 290.pdf