text: Coster grew up in Amsterdam in a large working-class family; he was the third child of Barend Coster, a blacksmith, and Aafje van der Mik. The Coster family valued education.  Ten of their children survived to adulthood and all received enough education to go onto middle-class professions.  From 1904 to 1908 Dirk went to the Teachers College in Haarlem, then was a teacher until 1913. With the aid of private support he was able to study mathematics and physics at the University of Leiden, first having passed the exams required for students who had no gymnasium education. In Leiden he was influenced by the inspiring lectures of Paul Ehrenfest, and in 1916 he obtained his M.Sc. degree. From 1916 to 1920 Coster was assistant of Lodewijk Siertsema and Wander de Haas at the Delft University of Technology, where in 1919 he obtained an Engineers degree in electrical engineering. In 1920 and 1921 he did research at Lund University under Manne Siegbahn, on X-ray spectroscopy of different elements. Costers thesis was on this subject, and he obtained his Ph.D. degree in 1922 in Leiden under Paul Ehrenfest; his thesis was entitled "Röntgenspectra en de atoomtheorie van Bohr" (X-ray spectra and Bohrs atomic theory).