This volume contains the proceedings of a postgraduate course for medical practitioners of various specialties. One purpose of the course was to provide factual data on developmental aspects of the brain and behaviour, and about the possible impact of several important categories of internal and environ­ mental factors upon neural development. Another purpose was to indicate the extent and the limitations of the methodology now available for the scientific approach of the study of the development of behaviour. In general the investigator is faced with methodological problems of two types, the proper definition and scoring of behavioural items, and the isolation of the different factors that contribute to a particular behaviour. An example of the latter is given in the very last paper, which is concerned with attempts at unravelling under experimental conditions the contributions made by various influences upon a single sequence of behaviour. The course was held in Leiden in November 1970, and was the third in a series of Boerhaave Courses instigated by the Dutch Growth Foundation. Previous subjects have been ''Somatic growth of the child'' (in 1964) and ''Human body composition'' (in 1967). The programme was planned in collaboration with Prof. Dr. H. H. van Gelderen, Dr. D. G. Lawrence, Prof. Dr. F. J. Monks, Prof. Dr. H. F. R. Prechtl and Prof. Dr. H. K. A. Visser. Financial support was given by the pharmaceutical firms Philips-Duphar, Sandoz and Specia, and by the Dutch Growth Foundation. Major editorial contributions were made by Anneke Bot.