This book constitutes the refereed proceedings of the 23rd Conference on Foundations of Software Technology and Theoretical Computer Science, FST TCS 2003, held in Mumbai, India in December 2003.The 23 revised full papers presented together with 4 invited papers and the abstract of an invited paper were carefully reviewed and selected from 160 submissions. A broad variety of current topics from the theory of computing are addressed, rauging from algorithmics and discrete mathematics to logics and programming theory.