This paper deals with the representation of the conceptual structure of mental models of dynamic systems (MMDS). Heretofore, this structure has not been consistently defined. Consequently, studies about MMDS continue to use different conceptual structures to measure mental models. Even such properties as feedback loops and delays, which lie at the core of dynamic systems, are often not considered. This situation leads to incompatible findings and stagnating research. We review the literature about mental models in the field of system dynamics. In addition, we refer to dynamic systems theory as the mathematical basis for system dynamics to complement and validate our conceptual structure. One may conclude that most of the existing mental model studies measure only parts of the structure that we propose. The paper's contribution is to elaborate the conceptual structure of an MMDS and to use this structure to operationally enhance the definition of an MMDS. Copyright © 2012 System Dynamics Society