Light-curing units (LCUs) are handheld dental devices that emit blue light (≈400–500 nm) to photopolymerize resin-based composites, adhesives, cements, and sealants. Modern LED LCUs, especially polywave types, are preferred for activating multiple photoinitiators efficiently. Effective curing depends on adequate radiant exposure (J/cm²), spectral match to the material, tip diameter, and proper technique, not just high irradiance.