The chemistry behind the development of technetium-99m (Tc-99m) complexes useful for determining renal (kidney) function is discussed. The design of these compounds so that they will be (1) stable in vivo, (2) actively taken up by the kidney and (3) excreted from the body without being metabolized is also discussed. Such issues as the purity of the product (>90% yield required), in vitro stability, and the requirement for ease of preparation are also mentioned. The development and chemistry of commercialized Tc-99m renal function radiopharmaceuticals are covered as are the interplay of the chemistry of Tc-99m and it macroscopic daughter isotope, Tc-99g. Basic technetium chemistry as well as renal physiology and function are also discussed briefly. An attempt is made to highlight areas where further work is needed.