The study deals with an existential role of preunderstandings in the works of the leading founders of quantum theory. First, the study introduces Heidegger’s hermeneutic-phenomenological approach to the concept of understanding. Second, this concept is applied to various ways of thinking with which physicists (Planck, Einstein, de Broglie, Rutherford, Bohr, Heisenberg and Schrödinger) approached the solutions of scientific puzzles and constructions of new assumptions in the development of quantum theory. The article seeks to contribute to the debate in the methodology (...) and philosophy of science and points out that the role of preunderstandings is still underestimated. (shrink)