This chapter provides a comprehensive exploration of thermal management in microelectronics systems. With the rise of high-performance computing, artificial intelligence applications, large language models, and power-hungry data centers, effective thermal management has become increasingly vital. The chapter begins with an introduction to fundamental heat transfer mechanisms: conduction, convection, and radiation. Both air and liquid cooling, as well as passive and active methods, are discussed, with a focus on innovations such as heat pipes, thermoelectric cooling, and advanced heat sink designs. The application of thermal resistance networks is explored. Compact and reduced-order models, including boundary-condition-independent compact thermal models and hybrid thermal models, are presented. Numerous case studies are included, addressing thermal design in power devices, multi-chip modules, and the impact of thermal balls. Small heat pipes are examined in detail, covering design principles, material considerations, and fabrication methods. The emerging trend of immersion cooling is also discussed. Finally, the chapter examines microchannel cooling systems, capable of dissipating heat fluxes exceeding 500 W/cm2, and their critical role in high-power applications such as SiC MOSFET power modules.