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Contactless sensing utilizes pervasive wireless signals (e.g., Wi-Fi, 4G/5G) to sense the context information of human targets. It achieves non-intrusive human sensing without requiring the targets to wear any sensors. Contactless wireless sensing has enabled a large range of applications including gesture recognition, activity identification, and vital sign monitoring. To analyze the sensing performance and guide the system design, theoretical models which can help people understand the underlying mechanisms are critical. In this chapter, we introduce the Fresnel Zone based theories for contactless sensing leveraging signal propagation models in the air. The sensing theories consist of reflection model and diffraction model. These two models respectively characterize the relationship between the target movements and corresponding signal variations when the target is located near to and far away from …