This chapter delves into the concept of mobility in mechanical systems, presenting both its theoretical foundations and practical applications. It begins with an explanation of kinematic pair connectivity and constraints, laying the groundwork for understanding how links and joints govern movement. The Gruebler-Kutzbach criterion is introduced as a pivotal equation for calculating degrees of freedom in planar and spatial mechanisms. The chapter discusses how different types of joints—revolute, prismatic, and higher pairs—affect system mobility, and includes tabulated notations for allowed and constrained motions. Special considerations, such as singularities, redundant constraints, and self-alignment, are briefly introduced, highlighting potential design complications. Through a blend of theoretical equations and illustrative figures, the reader gains insight into how mobility impacts mechanical behavior. The chapter ends with practical exercises involving mobility determination in various mechanisms, reinforcing analytical skills essential for mechanism synthesis and design.