This volume contains the proceedings of ICTAC 2005, the second ICTAC, International Colloquium on Theoretical Aspects of Computing. ICTAC 2005 took place in Hanoi, Vietnam, October 17-21, 2005. ICTAC was founded by the International Institute for Software Technology of the United Nations University (UNU-IIST) to serve as a forum for practiti- ers, lecturers and researchers from academia, industry and government who are interested in theoretical aspects of computing and rigorous approaches to so- ware engineering. The colloquium is aimed particularly, but not exclusively, at participants from developing countries. We believe that this will help developing countries to strengthen their research, teaching and development in computer science and engineering, improve the links between developing countries and developed countries, and establish collaboration in research and education. By providingavenueforthediscussionofcommonproblemsandtheirsolutions, and for the exchangeof experiencesand ideas, this colloquiumsupportsresearchand development in computer science and software technology. ICTAC is attracting more and more attention from more and more countries.