Ball milling has emerged as a versatile and sustainable mechanochemical route for the synthesis of nanomaterials with tailored structural and functional properties. By controlling a few key parameters—milling time, speed, ball-to-powder ratio, and atmosphere—this technique enables precise manipulation of crystallite size, morphology, and defect structure across diverse material systems, including oxides, alloys, composites, and amorphous phases. Its solid-state operation, scalability, and minimal solvent use position it as an environmentally responsible alternative to conventional synthesis routes. This chapter reviews the underlying mechanisms, critical process variables, and recent advances that establish ball milling as a predictive, energy-efficient platform for designing nanomaterials applied in sustainable energy conversion and environmental remediation.