Inductively coupled plasma optical emission spectrometry (ICP-OES) has been applied to the precise and accurate multielemental analysis of a variety of sample types for many years. Because the ICP is capable of reaching temperatures as high as 10,000°K, compounds introduced into it are atomized, and the resulting atoms are excited and can be ionized, with the ions produced also being excited, which translates into thousands of emission lines to choose from for multielemental analysis. In fact, ionic emission is more sensitive than atomic emission for many elements. Here, we explore the fundamental operating principles of ICP-OES, starting with a description of the basic components of an ICP emission spectrometer: nebulization system, plasma source, wavelength dispersive system, and detector. The features and limitations of different spectrometers are discussed along with how their capabilities can be extended by using alternative sample introduction strategies, such as vapor generation, electrothermal vaporization, and laser ablation. Hyphenating ICP-OES to chromatography enhances the selectivity of the latter while allowing the application of ICP-OES to speciation analysis. Using flow injection to supply sample to the nebulization system allows an increase in sample throughput through discrete sample injection and also enables online matrix separation and analyte preconcentration, which improves detection limits while removing sources of interference. Other strategies to deal with interferences are also discussed, such as alternative calibration strategies or using a mixed-gas plasma instead of the conventional argon plasma. Examples of application from the literature demonstrate the wide range of areas that benefit from ICP-OES.