Tinbergen's (1963, Z. Tierpsychol. 20, 410–433) four questions have guided the conceptual thinking of behavioural biologists over five decades. This is partly based on a misunderstanding of the aims and virtue of this article. Tinbergen called attention to the fact that his classification of problems is somehow illogical. Here, an attempt is made to remove some of the inconsistency by reformulating the levels of analysis of biological (including behavioural) traits. Furthermore, important merits of Tinbergen's milestone paper that have remained largely unnoticed are put in perspective. Perhaps the most lasting of several functions of that article has been its pivotal role in establishing a new research discipline, behavioural ecology.