The Fricke chemical dosimeter was developed almost 100 years ago. Nevertheless, it continues to be widely used, especially in radiation dosimetry, and new applications have been developed in recent years. This chapter reviews the basic principles of the Fricke dosimeter, describes how it is used in practice, discusses its strengths and limitations, and reviews its most important applications. Fundamental parameters for Fricke dosimetry, such as the value of the chemical yield, G(Fe3+) and the extinction coefficient, are summarized. Although the best precision is obtained using glass vessels, it is now recognized that useful results can also be obtained using different plastic vessels such as polymethyl methacrylate, high-density polyethylene, acrylonitrile butadiene styrene, and others. This has opened up new applications such as secondary standards for 192Ir brachytherapy dosimetry and dosimetry for blood irradiation.