A decision is a preferred option or a course of actions chosen from among a set of alternatives based on certain criteria. Decision theories have been focused on static and single decision making techniques. However, a wide range of problems and real-world challenges require serial decision making methodologies and a theoretical framework. This paper introduces a new decision and operation theory, known as the decision grid, for modeling and supporting dynamic and serial decision making. The mathematical model of decision grid is introduced and the properties of decision grids are rigorously described. The formal treatment of serial decision makings with limited or unlimited trails are modeled by decision grids. Decision grids can be applied in a wide range of serial and dynamic decision making situations in management science, operation studies, cognitive science, sociology, economy, software engineering, systems engineering, political science, statistics, as well as everyday lives.