Prof. Dr. Simeon Oka, 80th Anniversary 56 Years of Scientific Activity, 20 Years Acting as the Editor-in-Chief of the Journal Thermal Science The Laboratory for Thermal Engineering and Energy, Vinca Institute of Nuclear Sciences, and the Society of Thermal Engineers of Serbia have the pleasure and honor to present this Special Issue of the journal Thermal Science dedicated to the celebration of the 80th anniversary of Prof. Dr. Simeon Oka, Editor-in-Chief, and to his 56 year-long scientific activity. This year we also mark the 20th anniversary of the publication of the journal Thermal Science, initiated by Prof. Simeon Oka, the Editor-in-Chief. Prof. Simeon Oka has profoundly influenced the scientific profile of the Laboratory for Thermal Engineering and Energy and its development during many years, and he has also influenced the professional and, to some extent, personal lives of many of us during our joint activity and co-operation over the past years. Acting upon his personal wish, we have invited a large number of internationally reputed scientists who have been in close research collaboration with Prof. Simeon Oka for many years, to send their contributions for this issue. Many of them have been personal friends of Prof. Simeon Oka since the days of their youth. We are deeply grateful to Prof. Bo Leckner, Acad. Vladimir Nakoryakov, Prof. Anatolii Burdukov, Prof. Viktor Terekhov, Prof. Arun Mujumdar, Prof. Edward Anthony, Acad. Gligor Kanevce, Prof. Jovan Jovanović, Prof. Mladen Stojiljković, and Prof. Vladimir Stevanović for their valuable contributions.