This document discusses corona phenomenon in overhead transmission lines. It defines corona as the ionization of air surrounding power conductors, which causes a faint violet glow. Critical disruptive voltage and factors affecting corona such as atmospheric conditions, conductor size and spacing are explained. Methods to reduce corona loss include increasing conductor size, using bundled or hollow conductors, corona rings, and increasing spacing. While corona causes power loss and interference, it also reduces voltage surges and electrostatic stresses.