This book contains invited essays in memory of Leonid Hurwicz spanning a large area of economic, social and other sciences where the implementation or enforcement of institutions and rules requires the design of effective mechanisms. The foundations of these articles are set by social choice concepts, game theory, Nash, Bayesian and Walrasian equilibria, complete and incomplete information. Besides in-depth treatments of well-established parts of mechanism and implementation theory, contributions on novel directions deal, for instance, with a quantum approach to game and decision making under uncertainty, digitalization, and the design of block chain for trading. The outstanding competence and reputation of the authors reflect the appreciation of the fundamental contributions and the lasting admiration of the personality and the work of Leonid Hurwicz.