The concept of sustainability is a concept in which present generations enjoy the inheritance of previous generations, provided the inheritance is unchanged for future generations. This concept pervades all the social spheres, as the awareness of the disturbance of the human environment is growing. The paper explores and presents a theoretical framework for sustainable development, placing an emphasis on sustainable tourism development. In order to measure sustainable development and sustainable tourism development, sustainability indicators are used, which give an impression of the sustainability of a particular segment of society. This paper measures indicators of sustainable development and sustainable tourism development of the Republic of Slovenia, and compares them with the same indicators of the Republic of Croatia in order to understand the concept of sustainability of these two observed countries. A comparative analysis concluded that the Republic of Slovenia has more positive indicators of sustainable development and sustainable tourism development than the Republic of Croatia.