The goal of the human factors engineer is to work within the systems engineering process to ensure that a Crew Centric Design approach is utilized throughout system design, development, fielding, sustainment, and retirement. To evaluate the human interface, human factors engineers must often start with a low fidelity mockup, or virtual model, of the intended design until a higher fidelity physical representation or the working hardware is available. Testing the Warrior-Machine Interface needs to begin early and continue throughout the Crew Centric Design process to ensure optimal soldier performance. This paper describes a Four Step Process to achieve this goal and how it has been applied to the ground combat vehicle programs. Using these four steps in the ground combat vehicle design process improved design decisions by including the user throughout the process either in virtual or real form, and applying the user’s operational requirements to drive the design.