The rapid evolution of radar technology has unlocked new possibilities in Human-Computer Interaction (HCI), offering robust sensing capabilities beyond traditional modalities like cameras or touchscreens. This chapter introduces the fundamental principles of radar sensing, including the physics behind electromagnetic wave propagation, signal reflection, and Doppler effects. It explores various types of radar systems such as continuous-wave, pulse-Doppler, and Frequency-Modulated Continuous Wave (FMCW) and their specific applications in HCI. Additionally, it discusses how radars’ unique ability to penetrate certain materials and operate under various lighting conditions makes it an invaluable tool for tracking subtle human movements, enabling advanced gesture recognition and environmental awareness systems.