The development of the energy system of Slovakia is closely related to development of the national economy and society. The present period determines the character and further direction of the whole energy system of Slovakia. On the basis of existing data we have made an attempt to compare the official governmental alternative of development of the Slovak energy system based on support of nuclear energy with its alternative coming out of the principles of sustainability that is presented mainly by non-governmental organizations. The transition from the present unsustainable state and development of the Slovak energetic to sustainable one is supposed to be a long-lasting process presuming large structural changes in the economy of the country, in the whole energy system from production through transformation up to final consumption. In the next decades among the key factors influencing the sustainable or unsustainable direction of the energy system of Slovakia are: final decision to put in operation the nuclear power station in Mochovce, successes in application of energy, savings programmes and support of exploitation of domestic renewable energy, resources.