For the first time the revision of the "Guideline for radiation protection in medicine" defines extensive quality control procedures for radiopharmaceuticals. The principles of the 99Mo/99mTc-generator, the preparation of 99mTc-radiopharmaceuticals and the origin of the most frequent radiochemical impurities are illustrated. An introduction into the theory of thin layer chromatography is combined with a listing of radiochromatographic systems. This enables the determination of the radiochemical purity of the most important radiopharmaceuticals in clinical routine.