Neural networks, also called artificial neural networks (ANNs), are important techniques for performing forward/inverse modeling for microwave active/passive components to enhance a circuit design. With measured or simulated data of microwave devices, ANNs can be trained to learn relevant microwave relationships, which could be otherwise computationally expensive or for which efficient analytical formulas are not available. By training an ANN using data from electromagnetic (EM)/physics simulations, one can use the trained ANN as models for microwave circuits to replace the EM/physics models, which are typically CPU intensive, to significantly accelerate circuit design with EM/physics‐level accuracies. Fundamental concepts of the ANN structure and training, knowledge‐based neural networks, automated model generation, neuro‐transfer function modeling, deep neural network modeling, neural network‐based inverse modeling, and the use for EM/multiphysics design optimizations are described here.