The central thrust of this book is that geochemical data can be used to identify and interpret geological processes in igneous, metamorphic and sedimentary rocks. This chapter categorises geochemical data into major element oxides, trace elements, radiogenic isotopes and stable isotopes. The text discusses the main processes which control the chemical composition of planetary bodies, which operate in igneous and metamorphic rocks and at the Earth’s surface and describes the main analytical methods currently in use. These include the methods of X-ray fluorescence, mass spectrometry and inductively coupled plasma mass spectrometry for both whole-rock analysis and in situ micro-analysis. Sampling protocols are briefly described and the choice of a suitable analytical method is discussed. Potential sources of error in geochemical analysis are identified and discussed.