Theorists and experimentalists of diverse fields of research gather to discuss basic problems in quantum mechanics in the light of new technology. This volume collects 51 papers selected from over 200 papers published in the proceedings of the first four ISQMs (since 1983) by distinguished scientists. It includes articles by C.N. Yang, J.A. Wheeler, Y. Nambu, L. Esaki and M.P.A. Fisher, to name just a few, and contains topics ranging from quantum measurements to quantum cosmology.