Professor Lech Wojtczak spent his entire scientific life at the Nencki Institute of Experimental Biology PAS, where he worked since 1947. He is the author or co-author of two hundred scientific papers, promoted 15 doctors of science, was the Full Member of PAS and has been awarded with several prizes and honors. Among his students are three successive directors of the Nencki Institute, the authors of the present article. Scientific interests of Professor Lech Wojtczak were always dealing with bioenergectics, a discipline that he led in Poland and Eastern-Central Europe. In particular, his studies focused on the role of fatty acids and their derivatives on bioenergetics, on the regulatory role of surface potential of biomembranes on enzymatic and transport activities, on the regulatory role of calcium and magnesium in mitochondria, on the role of free oxigen radicals in bioenergetics, etc. Apart from being a great scientist, Lech Wojtczak was also a fantastic teacher, and an excellent scientific supervisor. Being well recognized in the world, he was placing members of his research group in foreignlaboratories, as well as was sending tchem to courses and conferences. This was opening their minds to the world, and to other cultures, and allowed Lech Wojtczak to form from his collaborators the next generation of good scientists and future leaders. The list of the most important pupils of Professor Wojtczak is given in the article. Lech was also excellent in social contacts, and in creating a friendly atmosphere. Together with his wife Anna, they kept an open home, often inviting collaborators to parties that usually led to long scientific discussions. With the sudden death of Pofessor Lech Wojtczak Polish science suffered a great loss. This eminent researcher, the father of Polish bioenergetics, but also a warm and modest person, will be dearly missed.