In the late sixties, the definition of ALGOL 68 I 11 , for a long time calledALGOL X, reached some stability. It is at that period (1967) our team started theproject of writing a compiler for that language. We had two goals in mind : to make significant research in the field of compiler methodology, to point out the special difficulties encountered in the design of the compilerand thus possibly influence the definition of the language.This book is concerned with the first goal only ; ALGOL 68 should be considereda support to explain and develop compiling principles and techniques.The whole book is directly based on the actual compiler we have written for theElectrologica-X8 computer ; this compiler has been operational since early 1973oSince May 1975, it is available on the ''BS-com~uter'', the Philips prototype developedby MBLE and which is at the origin of the UNIDATA 7720. In fact, the X8 has be~nmicroprogra~ed on the BS ; it is worthwhile to mention that microprogrammingdid not introduce any significant loss in efficiency.