This chapter comprehensively examines Earth’s atmosphere, detailing its origin, composition, structure, and dynamic behavior. This chapter begins with the formation of the atmosphere during the Earth’s early history, tracing its evolution from the primordial hydrogen–helium envelope to the second and third atmospheres shaped by volcanic outgassing and biological processes. The early atmosphere, dominated by volcanic gases, eventually transitioned to one capable of supporting life, marked by the emergence of oxygen produced by photosynthetic organisms. This chapter emphasizes the critical role of the atmosphere in maintaining the planet’s energy balance, regulating temperature, and enabling life. Key topics include the chemical composition of the atmosphere, with a focus on both constant and variable gases, such as nitrogen, oxygen, carbon dioxide, and water vapor, and their influence on the Earth’s climate system. The structure of the atmosphere is explored through its vertical stratification, including the distinct layers: the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. Attention is also given to the behavior of water vapor, which varies significantly across different regions and altitudes, and its crucial role in the greenhouse effect. Further discussion delves into the ionosphere and magnetosphere, exploring their functions in radiation propagation and atmospheric dynamics. This chapter concludes by exploring the molecular behavior of atmospheric components at various altitudes, highlighting the concept of the exosphere and the process by which molecules may escape Earth’s gravitational field. Overall, this chapter underscores the complexity and importance of the atmosphere in supporting life and shaping the planet’s climate.