Abstract: In this graduate thesis the model of the natural gas market in Croatia, and the business practices and strategies of the gas transmission system operator (Plinacro) and the gas market operator (HROTE) has been described. After January the first 2010 a new model of the gas market was applied, and it changed the relationships between the participants on the market. A new term was introduced - ''Gas year'' (Plinska Godina), the way delivery of gas is contracted was changed, a possibility of changing the supplier and time dynamics of reserving transport capacity was introduced. Also a model balance groups inside of which gas quantities put in or taken out of the transport system can be balanced. The head of each balance group is responsible for balancing his own group, while Plinacro as the gas transmission system operator is responsible for the operational balance of the system as a whole. Plinacro has been given a permit to conduct natural gas transport operations on a term of 30 years as a public service and a regulated energy sector function. Commercial balancing of the system is conducted by the gas market operator (HROTE) by means of collecting bids for energy of balance from service providers and accounting deviations of the balance group and the activated balance energy. HROTE as an energy subject has two energy licenses (one for organizing the electrical energy market, and another one for the natural gas market). These activities are conducted by HROTE as public services under the supervision of Croatian energy regulatory agency (HERA). Balance energy appeared together with market liberalization while in the future a market for energy balance is expected.