Monazite is a light rare earth element (LREE)-rich phosphate mineral that occurs as a trace phase in a wide variety of rock types, where it forms in response to different geological processes from igneous crystallisation, very low- to high-temperature metamorphism, and hydrothermal mineralisation. Monazite is also an ore mineral in some REE-deposits. Owing to its physical and isotopic robustness, high blocking temperature for U-Th-Pb systematics, and widespread occurrence, monazite has become a reliable and versatile geochronometer routinely employed in dating magmatic and higher-grade metamorphic rocks. More recently, it has been increasingly identified in hydrothermal mineral deposits and very low-grade metasedimentary rocks. The ability of monazite to grow at low temperatures can lead to complex age distributions reflecting xenocrystic or detrital grains and multiple episodes of metamorphic or hydrothermal growth. In situ, microbeam analytical techniques, such as SIMS, EPMA and LA-ICP-MS, can provide ages for these low-temperature events. However, textural characterisation and geochemical analysis are required to decipher the petrogenesis of monazite and interpret the meaning of resultant age data. In this paper, we review the petrographic and geochemical characteristics of monazite with well-constrained geological histories, to provide criteria to differentiate monazite that formed in different geological environments. In cases where multiple generations of monazite are present in a sample, differences in chemical composition can often be used as a valuable indicator of origin complementing textural and morphological discrimination. In general, there is a positive correlation between Th contents in monazite and the temperature of the environment in which it crystallised. Magmatic monazite typically contains >3 wt% ThO2, as does monazite in high-grade (T > similar to 500 degrees C) metamorphosed rocks. The Th/U ratios of high-T monazites are typically in the range 10-100. Thorium contents of low-T (<350 degrees C) hydrothermal and metamorphic monazites rarely exceed 3 wt% (mostly <1 wt%, mean 0.6 wt% from data compiled in this study), and their Th/U ratios largely fall between 0.1 and 10. Monazite from carbonatites is characterised by very low U (<30 ppm) and anomalously high Th/U (>300). Thorium content and Th/U ratio of monazite can be used as indicators for distinguishing between low-T metamorphic and hydrothermal, magmatic and high-T metamorphic, and carbonatite-related grains. In comparison with igneous and high-T metamorphic monazites, low-T metamorphic and hydrothermal monazites tend to have depleted Ca and Y, but elevated total REE contents, features which are potentially useful discriminators. On chondrite-normalised REE plots, igneous monazite typically displays a prominent negative Eu anomaly, which is less pronounced in high-T metamorphic monazite. Europium anomalies in low-T metamorphic and hydrothermal monazite range from variably negative in most cases to slightly positive in rare cases. Furthermore, the REE distribution patterns of metamorphic and hydrothermal monazites are widely variable, apparently controlled by compositions of both the fluid phase and the host rock, as well as the P-T conditions under which they formed. The accurate interpretation of monazite age data requires petrographic characterisation combined with high-quality geochemical data.