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Petrology (from Greek: πέτρα, petra, rock; and λόγος, logos, knowledge) is a field of geology that focuses on the study of rocks and the conditions on which they form. The word lithology once was approximately synonymous with petrography, but today lithology is essentially a subdivision of petrology focusing on macroscopic hand-sample or outcrop-scale description of rocks.

There are three branches of petrology, corresponding to the three types of rocks: igneous, metamorphic, and sedimentary.

Petrology utilizes the classical fields of mineralogy, petrography, optical mineralogy, and chemical analyses to describe the composition and texture of rocks. Modern petrologists also include the principles of geochemistry and geophysics through the studies of geochemical trends and cycles and the use of thermodynamic data and experiments to better understand the origins of rocks.

• Igneous petrology focuses on the composition and texture of igneous rocks (rocks such as granite or basalt which have crystallized from molten rock or magma). Igneous rocks include volcanic and plutonic rocks.
• Sedimentary petrology focuses on the composition and texture of sedimentary rocks (rocks such as sandstone, shale, or limestone which consist of pieces or particles derived from other rocks or biological or chemical deposits, and are usually bound together in a matrix of finer material).
• Metamorphic petrology focuses on the composition and texture of metamorphic rocks (rocks such as slate, marble, gneiss, or schist which started out as sedimentary or igneous rocks but which have undergone chemical, mineralogical or textural changes due to extremes of pressure, temperature or both)
• Experimental petrology employs high-pressure, high-temperature apparatus to investigate the geochemistry and phase relations of natural or synthetic materials at elevated pressures and temperatures. Experiments are particularly useful for investigating rocks of the lower crust and upper mantle that rarely survive the journey to the surface in pristine condition. The work of experimental petrologists has laid a foundation on which modern understanding of igneous and metamorphic processes has been built.

• Suite - a collection of rock specimens from a given area, or it can refer to the succession of closely associated sedimentary strata or layers, especially a repeated sequence of layers.

• Bayly, M. Brian (1968) Introduction to petrology Prentice-Hall, Englewood Cliffs, NJ;
• Blatt, Harvey and Tracy, Robert J. (1996) Petrology: igneous, sedimentary, and metamorphic W.H. Freeman, New York, ISBN 0716724383 ;
• Dietrich, Richard Vincent and Skinner, Brian J. (1979) Rocks and rock minerals Wiley, New York, ISBN 0471029343 ;
• Fei, Yingwei; Bertka, Constance M. and Mysen, Bjorn O. (eds.) (1999) Mantle petrology: field observations and high-pressure experimentation The Geochemical Society, Houston, TX, ISBN 0941809056 ;
• Stanton, R. L. (1972) Ore petrology McGraw-Hill, New York, ISBN 0070608431 ;

• Important publications in petrology

• Atlas of Igneous and metamorphic rocks, minerals, and textures Geology Department, University of North Carolina;
• Metamorphic Petrology Database (MetPetDB) Department of Geology and Geophysics, Louisiana State University;
• Petrological Database of the Ocean Floor (PETDB) Center for International Earth Science Information Network (CIESIN), Columbia University;