This text was prepared as a monograph for the International Encyclopaedia for Labour Law and Industrial Relations. It is based on a more detailed work which appeared in French in 1970 and in Spanish in 1977. The material was brought up to date and recast to correspond to the type of monographs con tained in the Encyclopaedia, which were aimed at providing concise, but reasonably detailed information and analysis of national laws and practice. Thus indications concerning the historical background, important as they may be in the present case, as well as the discussion of a number of theoretical questions, have had to be considerably reduced. However, detailed, up-to date information is provided on the system of international labour standards and on the substantive provisions of the most important of these international instruments. As part of the Encyclopaedia for Labour Law and Industrial Relations, the present study will most probably reach those engaged in research in the field of labour law, as well as many employers' organisations and a large section of the trade union movement. However, it has been considered useful to publish the study also in book form to facilitate its use in wider circles such as university teachers and students, diplomats, politicians, international lawyers, and those engaged in daily trade union activities. Table of Contents List of Abbreviations 15 Introduction 17 CHAPTER I. HISTORICAL AND GENERAL BACKGROUND 17 § 1. Definition 17 §2. Historical development 17 §3.