Cesium formate (CsHCOO) is an inorganic salt composed of cesium cations and formate anions. It is a white, crystalline, and highly water-soluble solid. It is commercially available as a high-density aqueous solution. The primary industrial application of cesium formate is as a high-density drilling and completion fluid in the oil and gas industry. Its high solubility allows for the formulation of clear, solids-free brines with densities exceeding 2.3 g/cm³, which are used in high-pressure, high-temperature (HPHT) wells and for reservoir drilling-in operations to minimize formation damage. In materials science, it serves as a precursor for the synthesis of cesium-containing compounds, including those explored for perovskite solar cells (e.g., CsPbI3) and other advanced materials like cesium metal-organic frameworks (MOFs). It can also function as a source of cesium ions in chemical synthesis. Handling requires caution due to its alkaline nature and the potential health effects associated with soluble cesium salts. Its high density and specific ion effects also make it a subject of study in fundamental physical chemistry.