Geochemistry is frequently used in mineral or petroleum exploration and also in detection of contaminants in the environment. The geochemist studies the constitution of, and the chemical reactions in, the various components of planet earth. Advances in instrumentation have greatly improved the analytical capabilities in this field. The study of biogeochemical processes among atmosphere, biosphere, hydrosphere and lithosphere reveal only a part of the bigger picture of energy cycling in different time scales varying by orders of magnitude. Better understanding and appreciation of these phenomena is leading to more subtle practical applications.