Refractometry is the analytical method of measuring substances' refractive in order to, for example, assess their composition or purity. A refractometer is the instrument used to measure refractive index. This article describes the main characteristics and measurement principle of refractometers of various types. Their advantages and disadvantages in various applications are also described. Types of refractometers: Abbe refractometer (the first laboratory instrument for the precise determination of the refractive index of liquids), the hand refractometer (used by beekeepers to determine the water content in honey, by wine producers to determine the sugar content of the fruit and the grape must, and by aquarists to determine the salt content of water in marine aquaria.), the digital refractometer (an automatic determination and readout of the boundary line), automatic refractometer (the highest level of accuracy of measurements, it is possible to conduct studies of temperature and frequency dependencies), and process refractometer (can provide real-time measurement data process medium, have to function under very high pressures). Application: the food, sugar, pulp and paper, refining and chemical, and pharmaceutical industries.