Algebraic Groups and Arithmetic is an area in which major advances have been made in recent decades. The School of Mathematics of the Tata Institute of Fundamental Research has been one of the significant contributors to the progress, under the leadership of Professor M.S. Raghunathan. The Tata Institute organised a conference on the theme during 17-22 December 2001, on the occasion of Professor Raghunathan turning sixty. The conference received enthusiastic response, and there were lectures by several experts on forefront topics in the theme, including group-theoretic aspects, diophantine approximation, modular forms, representation theory, interactions with topology and geometry, dynamics on homogeneous spaces. This volume is a collection of papers emerging from the Conference. In addition to original papers by several leading mathematicians in the area, it also includes two expository papers on the work of Professor M.S. Raghunathan, by the late Professor Armand Borel and Professor Gopal Prasad, which had also been presented at the Conference.