In this graduate thesis the main task was to define and conduct the testing of heat meters. Heat meters are devices which measure the quantity of heating energy, which a heat carrier (usually water), takes or gives in a heat exchanging circle and heat meters show that heat in agreed legal units, usually kilowatt hour kWh. In introductory chapter heat meters are defined and their mode of operation is explained. In the second chapter existing Croatian legal regulations and laws are listed and explained. Third chapter defines the division of heat meters on: complete, combined and hybrid instruments. Subassemblies of combined instruments are listed and described in detail and they are the flow sensor, the temperature sensor pair and the calculator or a combination of these. In the third chapter there is also an overview of terms and definitions which is used in Croatian standards and other literature related to heat meters and testing of heat meters. Fourth chapter defines testing procedure at the first verification of heat meters. Purpose of that testing procedure is making sure that heat meters which are released in operation mode comply with the approved sample and regulations, i.e. that they have prescribed metrological properties within the limits of the maximum permissible errors as well as that they function correctly. All the provisions of the first verification test will also apply to the re-verification test. Fifth chapter defines the theory behind the determination of measurment uncertainty. Sixth chapter describes the procedure for testing heat meters on specific measuring lines installed in the Laboratory for Thermal Energy (LTE) at the Faculty of Mechanical Engineering and Naval Architecture in Zagreb, and is accepted as a national standard for testing and service of heat meters in the Republic of Croatia, which enables Croatia equal participation in European and world metrological tests in the subject area. This chapter also contains a schematic presentation of measuring lines, calculation of measurement uncertainty and proposals for calibration sheets. Seventh chapter, the concluding chapter contains a brief overview of the preparation of this graduate thesis and comments on the testing of heat meters.