The goal of this paper is to offer a clear and compact description of what ontology meant to the classical school of phenomenology, including the founder of the phenomenological movement, Edmund Husserl, and his original students: Adolf Reinach, Jean Hering, Hedwig Conrad-Martius and Edith Stein. Phenomenology as a theory and method was linked to the development of modern logic and the natural and human sciences in the first half of the twentieth century, and its ultimate purpose was to establish a dialogue between sciences and philosophy on new grounds, with ontology being the medium in which such a dialogue could take place.