This volume has been composed to commemorate the day, 25 years ago, that Dr. P. J. Verdoorn delivered his inaugural lecture after his appointment as professor of economics at the Netherlands School of Economics at Rotterdam. The prompt and enthusiastic reactions by several members of the profession on our request for a contribution illustrates the fact that Verdoorn is held in high esteem by those who worked with him and those who are acquainted with his work. We thank all the contributors for having helped us to realize our plan to honour Verdoorn with the publication of this 'liber amicorum'. Verdoorn was appointed part time professor of business statistics, market research and marketing in 1952. He held this post until 1969. In that year he started lectures closer related to his work as deputy director of the Central Planning Bureau: general economic research. He became full professor of macro-economic policy in 1975. Verdoorn showed a great versatility in all the fields on which he worked. Without exaggeration he may be characterised as one of the most many-sided economists in the Nether­ lands. However much the subjects he studied may have varied, the methods he applied show persistent characteristics: theories are formulated to yield testable hypotheses which, when they can be maintained, are made operatio­ nal for policy purposes. In all these phases of a research program Verdoorn showed a high level of inventivity. A combination of rigour and imagination enabled him to maintain a constant stream of high-quality publications.