The programming language field is certainly one of the most important subfields of computer science. It is rich in concepts, theories, and practical developments. The present paper attempts to trace the 25 year development of programming languages by means of a sequence of 30 milestones (languages and concepts) listed in more or less historical order. The first 13 milestones (M1-M13) are largely concerned with specific programming languages of the 1950's and 1960's such as Fortran, Algol 60, Cobol, Lisp, and Snobol 4. The next ten milestones (M14-M23) relate to concepts and theories in the programming language field such as formal language theory, language definition, program verification, semantics and abstraction. The remaining milestones (M24-M30) relate to the software engineering methodology of the 1970's and include a discussion of structured programming and the life cycle concept. This discussion of programming language development is far from complete and there are both practical developments such as special purpose languages and theoretical topics such as the lambda calculus which are not adequately covered. However, it is hoped that the discussion covers the principal concepts and languages in a reasonably nontrivial way and that it captures the sense of excitement and the enormous variety of activity that was characteristic of the programming language field during its first 25 years.