Implemented specifically as a method of theory development, computer simulation allows clarification of a theory and investigation of its implications. Using Petty and Cacioppo's (1986a, 1986b) Elaboration Likelihood Model (ELM), the use of simulation in formalizing a theory, testing the simulation model, and conducting simulation experiments is demonstrated. With formalization of the theory in the form of a block diagram, the entire pattern of causal effects in the ELM core statements becomes visible at a glance. The simulation model was tested through comparing simulated individuals' reactions to stimuli with the experimental and statistical observed reactions of real participants in experiments. The simulation experiments revealed a dynamic attitude shift in dependency on the development of processing intensity.