Integration of automatic engine Stop/Start systems in “conventional” drivetrains with 12V starters is a relatively cost-effective measure to reduce fuel consumption. Therefore, automatic engine Stop/Start systems are becoming more prevalent and increasing market share of such systems is predicted. A quick, reliable and consistent engine start behavior is essential for customer acceptance of these systems. The launch of the vehicle should not be compromised by the Stop/Start system, which implies that the engine start time and transmission readiness for transmitting torque should occur within the time the driver releases the brake pedal and de-presses the accelerator pedal. Comfort and NVH aspects will continue to play an important role for customer acceptance of these systems. Hence, the engine stop and re-start behavior should be imperceptible to the driver from both a tactile and acoustic standpoint.This paper describes the details of various powertrain calibration factors for the engine start process. The key phases of an engine start event are described in detail, and their influence on the vehicle vibration investigated. The engine stop behavior is analyzed with respect to crank position and possible engine “roll back” and its influence on vehicle vibration. Unconventional start systems such as assisted direct starts are investigated with respect to engine start time and associated NVH behavior. Case studies are utilized to illustrate the influence of cylinder pressure, spark, and injection system parameters on the engine start process. In addition, key parameters of torque converted equipped planetary automatic transmissions with respect to the engine start behavior are analyzed. Comparisons of different Stop/Start systems are conducted and their influence on engine start, launch delay, and “change-of-mind” engine re-start are summarized.