Don Mellenbergh (1938-2021) was Program Leader of the Psychological Methods group between 1982 and 1999. Mellenbergh obtained his Ph.D. at the University of Amsterdam in 1971, under the supervision of Adriaan de Groot. After that, he worked at Utrecht University for several years, where he left out of dissatisfaction with the reform of the academic curriculum which, in his view, had gotten out of hand. In 1975 he wrote a controversial note on the situation, De verloedering ener subfaculteit (The deterioration of a subfaculty), which gained national interest and was discussed in the Dutch parliament. He returned to Amsterdam, where he led the Psychological Methods group for almost two decades. Don Mellenbergh published many important contributions to psychometrics. He developed the concept of item bias, coining the distinction between uniform and non-uniform bias, and laid a cornerstone for the now standard unified view of psychometric models with his famous publication on Generalized Linear Item Response Theory. Perhaps even more important were his contributions to the social fabric of methodology. He was the founder of the Interuniversity Graduate School for Psychometrics and Sociometrics (IOPS), which became an internationally leading institute for graduate education in methodology. Mellenbergh acted as promotor of dozens of psychometricians, including Wim van der Linden, Henk Kelderman, and Denny Borsboom.