This chapter focuses exclusively on four-bar linkages, one of the most fundamental and widely used planar mechanisms. It introduces the different classifications of four-bar linkages—crank-rocker, double-crank, double-rocker—based on link lengths and motion behavior. The Grashof condition is presented to determine the possible type of motion within a given configuration. Various configurations and their input-output relationships are analyzed through geometric and trigonometric principles. Readers are guided through the loop-closure equation, a key tool for analyzing linkage positions and deriving transmission quality equation. The chapter highlights the versatility of the four-bar mechanism, noting its applications in tools, robotics, and manufacturing systems. Clear illustrations support the theoretical discussions, helping visualize link positions and angular constraints. Practical design implications, such as selecting appropriate link lengths to achieve continuous or oscillatory motion, are emphasized. Overall, the chapter provides a robust introduction to a central element of mechanism design.