Numerical modeling has emerged as an indispensable tool in the design and optimization of AlGaN/GaN High-Electron-Mobility Transistors (HEMTs). These devices, based on the AlGaN/GaN heterostructure, exhibit remarkable properties such as high electron mobility, wide bandgap, and high breakdown voltage, making them highly promising for high-power and high-frequency applications. However, the complex interplay between material properties, device geometry, and operating conditions poses significant challenges in understanding and predicting the device behavior solely through experimental techniques. To address these challenges, numerical modeling techniques have become vital for gaining deeper insights into the underlying physics, optimizing device performance, and guiding the design process. This chapter provides an overview of the numerical modeling approaches employed in the study of AlGaN/GaN HEMTs, with a specific focus on Density Functional Theory (DFT), Finite Element Method (FEM), Monte Carlo simulation, and commercially available software packages such as COMSOL and Sentaurus TCAD. The chapter begins by introducing the AlGaN/GaN HEMT structure, highlighting its layered composition, including the AlGaN barrier, GaN channel, and various doping profiles. The unique characteristics of AlGaN/GaN HEMTs, such as the 2-Dimensional Electron Gas (2DEG) formed at the heterointerface and the polarization-induced charges, are discussed. These features contribute to the exceptional electronic properties of the devices but also introduce complexities that necessitate numerical modeling for comprehensive understanding.