This chapter provides an overview of the most important tracer agents for renal studies which have been used in the past or are in use. The extensive search for suitable renal radiopharmaceuticals can be understood from the viewpoint that the kidney is a vital organ responsible for the maintenance of composition of blood and extracellular fluid and deserves optimal diagnostic agents for its evaluation. Most radiopharmaceuticals proposed for effective renal plasma flow studies can be regarded, as substances with a hippurate-like behavior. The structural requirements for efficient secretion by the renal tubules have been studied in detail for organic anions by comparing the renal handling of a large number of carboxylic acids, mercurial diuretics, sulfonic acids and sulfonamides. Many 99mtechnetium-complexes have been reported to show significant uptake and retention in the kidneys and were proposed as potential radiopharmaceuticals for morphological kidney imaging or determination of functional renal mass.