Monitoring the health of vibrating gears is important to ensure proper operation especially in potentially life-threatening structures, such as helicopters, nuclear power plants, and uninterruptible power supply transitions in hospitals. The most common monitoring technique is casing mounted accelerometers to measure vibration. In contrast, during the last few years acoustic monitoring techniques have also provided a few diagnostic methods for gear failure. Current diagnostic methods to indicate improper gear behavior use either existing vibration data, recorded from defective gear systems, or modern dynamic models predicting gear failure behavior. This thesis uses dynamic models to indicate, predict, and diagnose healthy and unhealthy gear systems. Influence of Tip Relief on contact forces are introduced for a decent understanding of gear dynamics followed by evaluation of common gear failure mechanisms. Two software systems were used to model gear failure: Adams®, a vibration based software that uses a rigid-elastic model for multi-body dynamics, and LSDYNA ®, a transient dynamic finite element solver, capable of solving acoustic problems with the boundary element method. Results describe tooth loads along the line of contact with respect to different Tip Reliefs and contact ratios. Gear failure is examined using a Fast Fourier Transformation to characterize patterns that can be used to diagnose unhealthy gear systems. Agreement of experimental results validates theoretical predictions of analytical and numerical solutions of gear failure especially of tooth breakage.