Neural Networks are information processing systems inspired by the ability of human brain to learn from observations and to generalize by abstraction. It has been used in diverse fields such as pattern recognition, speech processing, control, medical applications and more. In recent years, it has emerged as an attractive vehicle in the RF/microwave CAD community to address the challenges in high-frequency electronic modeling and design. Neural networks can learn and generalize from data allowing model development even when component formulas are unavailable. Neural network models are universal approximators allowing re-use of the same modeling technology for both linear and nonlinear problems and at both device and circuit levels.