Decision aids are computer systems intended to help decision makers select from a set of alternatives when considering multiple criteria. There are a number of basic ideas behind such systems, including multiattribute utility analysis, analytic hierarchy process, and outranking. Some systems focused on identification of decision maker preference functions. Recently there have been a number of systems focusing on the idea of enhancing decision maker learning. This paper reviews the fundamental ideas of a number of decision aids and considers the cognitive effort on the part of decision makers required to use each system. Recent empirical studies from a number of sources are reviewed and evaluated for support of theories about this cognitive effort, as well as how learning systems are implemented.