Technetium-99m labeled radiopharmaceuticals are the currently accepted agents of choice for skeletal imaging. Their introduction in 1971 literally initiated a new era in clinical bone scanning. The development of techniques for reducing Tc(VII) with Sn(II) provided the means for complexing this useful radionuclide with various phosphorus-containing compounds which were already known to be avid bone seekers. Long chain polyphosphates were widely used at first, but have been superceded by pyrophosphate and its organic analogs, the diphosphonates. Pyrophosphate is characterized chemically by P--O--P bonds, and the diphosphonates by P--C--P bonds. The chemical forms of their complexes with tin and technetium are not known, but they behave in many respects as weak chelates. Labeling efficiencies for 99mTc of 95% or better are routinely obtainable with both "in house" preparations and commercial kits. Proper molar concentrations and ratios of phosphorus-compound to tin are necessary for both for good labeling and to achieve optimum tissue distribution. Unreacted TcO4- and reduced unbound 99mTc are both potential contaminants in these preparations and must be considered in radiochemical quality control. In vivo tissue distribution and kinetics of the 99mTc-Sn-phosphorus compounds differ with details of preparation, category of agent, and clinical status of the patient. Blood clearance is multi-exponential, with skeletal uptake and urinary clearance accounting for most of the activity. Scanning may be started in 2 1/2 to 4 hr, at which time skeletal activity is on the order of 40 to 50% of the injected dose. The primary indication for bone scanning remains the detection of metastases from extraskeletal malignancies, and the 99mTc labeled agents are more sensitive than either radiographs or Fluorine-18 for demonstrating active lesions. In addition, many new applications in evaluating benign bone disease have widened the clinical scope of skeletal imaging which is rapidly becoming one of the most important studies in nuclear medicine.