The advent of powerful wearable devices and smartphones has enabled a new generation of “in-the-wild” user studies, adaptive behavioral intervention strategies, and context measurement. Though numerous proof-of-concept studies continue to push the limitations of what a behavioral scientist can do with these technologies, there remains a major methodological roadblock separating behavioral theory and application. Avatar-user interaction theory, for example, is not well defined in its formulation, and thus guidelines for intervention designers depend on heuristic methods and designer intuition. Computational modeling has been slow to move into behavioral science in general, but a growing population of behavioral scientists recognize this shortcoming and are eager to apply new technology to their work. In order to help close this disciplinary rift between systems engineers and behavioral scientists, human-computer interaction principles must be applied to make the seemingly inaccessible “magic” of modeling and simulation techniques accessible to behavioral scientists. Thus, this dissertation presents formative work to help bring engineering methodology to human behavior modeling and simulation. Using theories of avatar-user interaction theory, physical activity regulation, and “information overload” as applications to drive toolkit design, usability considerations and interface needed to connect behavioral scientists with dynamical systems modeling are explored. A number of challenges unique to the modeling of human behavior and quirks of extant modeling efforts in behavioral science mean that existing modeling tools do not satisfy the needs of the community, and a novel design to address these shortcomings is presented. Exploration of the fundamental design questions which arise from application of engineering principles to this unique problem will produce quality publications in software engineering, HCI, and behavioral science. Furthermore, both the “behaviorSim” toolkit and the innovative inclusion of modeling and simulation represent significant contributions to the development and application of human behavioral theory.