In 1951 the AMS published the Compendium of Meteorology in order to “take stock of the present position of meteorology, to summarize and appraise the knowledge which untiring research has been able to wrest from nature during the past years, and to indicate the avenues of further study and research which need to be explored in order to extend the frontiers of our knowledge.” The mammoth volume (1334 pages) consisted of reviews for 25 “specialties” in the field of meteorology. In his review of climatology (“Climate-The Synthesis of Weather”), C. S. Durst made the following observations: (1) “…climatology as presently practiced is primarily a statistical study without the basis of physical understanding, which is essential for progress.”, (2) “…there has not generally been an insistence on knowledge of the physical reasoning which must underlie climatology.” and (3) “…there has been a woeful tendency to the use of the bones of bare statistics and mean values without the flesh of physical understanding.” Durst went on to say “As I see them, the essential needs of climatology are in the first place a re-orientation of the expression of climate and of the teaching of climate, and secondly the explanation of climate as a physical and dynamical phenomenon.” The half century since the publication of this review has witnessed a remarkable evolution from the narrow, statistically oriented focus of climatology on land surface meteorological variables into the vastly more comprehensive interdisciplinary enterprise of climate system science. The broadened perspective of the global climate system includes many major aspects of what were considered other meteorological “specialties” at the time of the publication of the Compendium, e.g., upper air analysis and the atmospheric general circulation, and also encompasses major aspects of the associated hydrological and oceanic sciences. Key aspects of this evolution are the perception of climate as variable and a non-stationary process, the development of dynamically- and physically-based climate diagnostics, and numerical climate model simulations. A number of important milestones in the development of climate system science will be reviewed briefly in this presentation, with emphasis on those areas of research in which the speaker has participated.