Petrology (from the Greek πέτρος, pétros, "rock" and λόγος, lógos, "subject matter", see -logy) is the branch of geology that studies the origin, composition, distribution and structure of rocks.

Lithology was once approximately synonymous with petrography, but in current usage, lithology focuses on macroscopic hand-sample or outcrop-scale description of rocks while petrography is the speciality that deals with microscopic details.