 Although Brouwer’s development of intuitionism played an important role in the foundational debate among mathematicians at the beginning of the 20th century, the far reaching implications of his philosophy for mathematics became only apparent after many years of research. The two most characteristic properties of intuitionism are the logical principles of reasoning that it allows in proofs and the full conception of the intuitionistic continuum. Only as far as the latter is concerned, intuitionism becomes incomparable with classical mathematics. In this entry the focus is on those principles of intuitionism that set it apart from other mathematical disciplines, and therefore its other constructive aspects will be treated in less detail. 