This chapter introduces the fundamental theories governing fluid mechanics in the atmospheric boundary layer, including the basic equations of motion (Sect. 2.1) and surface layer and Ekman layer theories (Sect. 2.2). However, relying solely on these theories and equations is often insufficient to construct mathematical models for variables of interest (such as the mean wind profile). Instead, concepts of atmospheric stability (Sect. 2.3) and similarity theory (Sect. 2.4) offer frameworks for developing observation-based models. These similarity relationships form the basis of modern boundary-layer meteorology. Readers primarily interested in practical applications, such as design of civil structures against wind actions, wind energy resource assessment, and pollutant dispersion modeling, may opt to skip this chapter and proceed directly to Chap.  3 .