Hugo de Vries was born in 1848. At about the turn of the 19th century he was professor of botany and director of the Hortus Botanicus at the University of Amsterdam. On the 150th anniversary of his birth, a symposium was held in Amsterdam on 27 March 1998. The majority of the lectures have been elaborated into the papers published in this special issue of Acta Botanica Neerlandica. The editors are indebted to Ferry Bouman and Erik Zevenhuizen for their guest editorship. Hugo de Vries was strongly influenced by the plant physiologist Julius Sachs and the naturalist Charles Darwin. Following Sachs, De Vries dedicated himself for almost two decades to plant physiological research. Through his writings and the lectures to his students he introduced the experimental approach to biology into The Netherlands. In the 1890s his interest gradually shifted towards heredity and evolution. De Vries remained faithful to the way of working he had developed in his physiological research, and he became one of the first to try to solve problems in heredity and evolution using an experimental approach. His conviction that evolution is not a gradual and extremely slow process, but one that proceeds by minute, observable steps caused by genetic changes, culminated in his mutation theory published in 1900. The theory was based on De Vries’ profound knowledge of all aspects of plant biology and plant breeding, and was supported by ample evidence from an enormous amount of experiments.