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...