The research is based on the application of international sustainability indicators within the IBNET Apgar methodology and WUVI system sustainability indicators. Theoretical and practical assumptions for determining the sustainability of water supply systems have been elaborated. In this context, the IBNET Apgar and WUVI methodology was applied, and the IBNET Apgar and WUVI indicators were calculated for companies in the water supply business of the Republic of Croatia for the period from 2011 to 2018.The research sample consists of all large and medium-sized enterprises according to the classification based on current accounting regulations. Data on business operations, water prices, quantities of pumped and delivered water, as well as data on the length of the water supply network were obtained from relevant databases at the national level. The dissertation presents the results of research based on statistical analyses to determine the direction and intensity of the interdependence of business sustainability indicators and IBNET Apgar results. The correlation between the IBNET Apgar and WUVI indicators and dominant costs was analysed, as was the state and sustainability of the company's operations assessed by quantitative research methods. The IBNET Apgar methodology has proven to be a useful methodology for determining the sustainability of water supply companies in the Republic of Croatia. The research proved the connection between the management of dominant costs in the performance of water supply activities and long-term business sustainability. Less investment in network expansion and increased investment in maintenance, reconstruction, and rehabilitation of infrastructure, result in lower water losses. By optimizing costs and allocating investments in new infrastructure while at the same time investing in the maintenance of the existing infrastructure, losses are reduced, and new investments make a financially profitable, environmentally responsible, and socially sensitive model of cost allocation. The research established how and to what extent certain types of costs affect long-term business and profit of the company. Furthermore, it was determined which types of costs can be used to predict the long-term sustainable operation of companies in the water supply industry.