The Decision Systems Theory presents a set of concepts and laws that contribute to explain the cognitive processes in which human systems value and evaluate their reality in order to choose among different options of manifestation. All the behaviors that human systems execute, whether individual or collective, are the product of a decision process. Based on the Mental Systems Theory, this book introduces the concept of decision system. The decision system is the mental program through which human systems determine their options of manifestation, which are restricted or enabled by certain endogenous and exogenous factors, and select the one they consider the most valuable, according to their structure of preferences. This model seeks to explain why the multiple and different types of behaviors that human systems can develop are generated in certain circumstances. Throughout this work, we will explore the variables that intervene in the gestation of different human decisions, within a rigorous conceptual framework.