Now it is evident that our joint decision to publish Special issues with selected papers presented at SDEWES Dubrovnik Conferences has been a far looking decision which from the year 2008 greatly contributed in advertising idea of sustainable development in the South East European (SEE) Countries, and especially in those aiming to become member of the European Union. From year to year and from one SDEWES Conference to the next one, number of papers presented by authors from SEE Countries, is growing. We believe that significant influence on this trend comes from our Special issues, also, in which we publish every two years, papers selected by reputed Guest editors, devoted mainly to the most important energy problems, common for all SEE Countries. In this issue, in Part one, nineties papers, mainly written by authors from SEE Countries, presented at 10th SDEWES Dubrovnik Conference, held from September 27th to October 2nd, 2015, are selected. As Guest editors pointed out in their excellent introductory paper, most of the papers selected are devoted to the problem: how in specific country achieve optimal energy mix.