Understanding upper extremity functional anatomy is fundamental for the rehabilitation of patients with tetraplegia, as improvement in upper limb function represents their highest priority for recovery. This chapter examines the anatomical and functional relationships of three key anatomical regions—the shoulder complex, elbow complex, and wrist–hand unit—with particular attention to their nerve supply and musculature. The text analyzes how the neurological level of injury affects upper extremity function and explores opportunities for functional enhancement through targeted rehabilitation strategies based on anatomical and kinesiological principles. The shoulder complex’s unique mobility enables hand positioning throughout the workspace while maintaining stability for both power and precision tasks. This remarkable versatility stems from the coordinated action of multiple joints and complex muscular systems that provide both mobility and stability. The elbow complex provides essential stability and positioning capability through its dual role in flexion-extension and pronation-supination movements, while the wrist–hand unit serves as the primary effector for manipulation and grasping. Special emphasis is placed on the integration of these three complexes as a unified functional system. The chapter examines nerve supply patterns and muscular arrangements that enable coordinated movement across multiple joints, including the complex interactions between extrinsic and intrinsic muscle systems. This integrated approach considers both the structural and neural components that contribute to upper limb function, acknowledging the importance of proprioceptive feedback and motor control in coordinated movement. This comprehensive knowledge supports the development of goal-oriented rehabilitation programs and appropriate selection of orthotic devices and adaptive equipment, ultimately aiming to maximize functional independence and quality of life for individuals with tetraplegia.