Amyloidosis and sarcoidosis are systemic diseases that affect multiple organ systems. Accurate diagnosis of cardiac amyloidosis and sarcoidosis is particularly important because cardiac involvement can be fatal. Amyloidosis is characterized by the deposition of amyloid fibrils, and cardiac amyloidosis is classified into amyloid immunoglobulin light chain (AL) and amyloid transthyretin (ATTR) types. Radionuclide tracers for amyloidosis include (a) bone tracers, (b) amyloid-directed molecules, and (c) PET amyloid agents. Bone tracers are particularly sensitive in detection of ATTR type amyloidosis, whereas PET amyloid agents show a higher affinity for the AL type. In sarcoidosis, gallium 67 (67Ga) citrate scintigraphy and fluorine 18 (18F) fluorodeoxyglucose (FDG) PET are pivotal to diagnosis of cardiac sarcoidosis, and 18F-FDG PET/CT has particularly high efficacy in detection of sarcoidosis and monitoring of response to therapy. A major limitation of 18F-FDG is physiologic uptake in the myocardium, which can remain in approximately 20% of patients even after elaborate preparation (eg, prolonged fasting >12-18 hours, modification to a high-fat and low-carbohydrate diet, and injection of unfractionated heparin). This limitation has led to a search for potential new tracers. Recently introduced tracers that show promise include those used in somatostatin receptor imaging and cellular proliferation imaging, which provide detectability as high as that for 18F-FDG without requiring dietary restrictions and have potential for monitoring disease activity. ©RSNA, 2020.