The Thirty-First International Workshop on Condensed Matter Theories (CMT31) held in Bangkok focused on the many roles played by ab initio theory, modeling, and high-performance computing in condensed matter and materials science, providing a forum for the discussion of recent advances and exploration of new problems. Fifty-six invited papers were presented, of which 38 appear as chapters in this volume. Reports of recent results generated lively debate on two-dimensional electron systems, the metal-insulator transition, dilute magnetic semiconductors, effects of disorder, magnetoresistence...