Oskar Becker’s work in the philosophy of mathematics makes an important contribution to the philosophical understanding of the constructivist program. Becker (1889-1964), a student of Edmund Husserl and an associate of Martin Heidegger, initially sought to ground a constructivist view of mathematics in Husserl’s transcendental phenomenology; subsequently he adopted a Heideggerian and existential view of mathematics that, he argued, would allow one to rescue large parts of classical mathematics from an intuitionist and constructive perspective. In his later writings he finally turned to a radically historicist interpretation of the constructivist program.