The recombination lifetime τ^r is a very effective parameter to characterize the purity of a material or a device, because it is sensitive to very low densities of contamination. Impurity densities as low as 10¹⁰ cm⁻³ can be detected. τ^rhas become a process and equipment characterization and evaluation tool. The various recombination mechanisms are discussed here. We show that surface recombination plays an important role in today's high purity Si and will become yet more important as bulk impurity densities in Si are reduced further. The dependence of lifetime on impurity energy level and minority carrier injection level is discussed. Concepts are stressed in the paper, with the necessary equations to clarify these concepts. Wherever possible, the concepts are augmented with experimental data, with particular emphasis on iron in silicon, because Fe is one of the most important impurities in Si today.