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 phenomena, ferromagnetic stripes, quantum Hall systems, strongly correlated Fermi systems, superconductivity, dilute fermionic and bosonic gases, nanostructured materials, plasma instabilities, quantum fluid mixtures, and helium in reduced geometries.