1.1. GENERAL The study of the prevalence of mental retardation presented in this book is especially concerned with assessing the prevalence within a number of age groups in the city of Amsterdam, i.e. in the groups of 10 and 13 year-olds - who together are called the children - and those of 16, 19, 25, 34, 40 and 50 years old-together called the older pupils. This study is to be seen as that part of the report the National Survey of the Mentally Handicapped which took place in Amsterdam in the years 1966-1968. The problems to be investigated were as follows: 1. How many mentally retarded are there in Amsterdam and what degree of mental retardation do they show? 2. What forms of special care are needed for these mentally retarded and for the families to which they belong? This work contains the answer to the first of the above questions. Chapter 2 gives a survey of the Amsterdam investigation, its back­ ground, preparation, pIan and execution. Chapter 3 deals with a number of definitions, the most important of which is the definition of mental retardation. The comparatively detailed discussion of the criteria used and their application emphasize especially the fact that the Amsterdam investigation has obvious explorative as· pects. This chapter must not, however, be regarded as a general ex­ position of mental retardation.