Role of mast cells in allergy had remained undetermined until the discovery of IgE in 1966. Then, IgE purified from many Liters of plasma, which had been donated from a patient with fatal myeloma, was distributed to researchers all over the world, and thus accelerated exploring the mechanisms involved in allergic reactions, particularly about the role of mast cells and basophils in the IgE-mediated reactions. Identification of mast cells as a progeny of a bone marrow hematopoietic stem cell in 1977 led us to successful in vitro culture of human mast cells. Along with the development of molecular biological techniques, the structure of the high affinity IgE receptor (FceRI) was determined in 1989. These findings and subsequent investigations brought deeper understanding of IgE-mediated allergic diseases in the past half century, especially where mast cells are involved. We have now even obtained the information about whole genome expression of FceRI-dependently activated mast cells. In sharp contrast to our comprehension of allergic diseases where IgE and mast cells are involved, the mechanisms involved in non-IgE-mediated allergic diseases or non-IgE-mediated phase of IgE-mediated diseases are almost left unsolved and are waiting for devoted investigators to reveal it.