"INTRODUCTION The author did not fully realize the immensity of the task when he volunteered to review the major publications of last 50 years on the subject of concrete durability. The Proceedings of the International Conference on Concrete Durability (I)@ which was held four years ago in Atlanta@ Georgia@ to honor Katherine and Bryant Mather@ contain well over 2000 pages covering more than 100 papers. In addition@ the proceedings of symposia sponsored by ASTM (2)@ AC1 (3)@ and RILEM (4@5)@ as well as eight international congresses on the chemistry of cement contain a wealth of information on the subject. For instance@ at the Eighth International Congress at Rio de Janeiro in 1986@ the state-of-the-art report on chemical aspects of durability alone contains a reference to 244 papers (6). Similar reports on the chemical aspects of durability presented at the Seventh Congress at Paris in 1980 (7)@ at the Fifth Congress at Tokyo in 1968 (8)@ at the Fourth Symposium at Washington D.C. in 1960 (9)@ and at the Third Symposium at London in 1952 (IO)@ contain references to 291@ 128@ 105@ and 82 papers@ respectively. Interestingly@ the proceedings of the Second Symposium at Stockholm in 1938 contain only one paragraph on the subject of chemical resistance@ and no other references to concrete durability. To the above-cited literature on the chemical aspects of concrete durability@ if one adds the published literature on deterioration of concrete due to freezing-thawing cycles and corrosion of reinforcing steel@ the exercise of doing a comprehensive review simply becomes unmanageable. Under these circumstances@ one can at best undertake a review of the previously published reviews. This is exactly what the author has done and reported in this paper. Besides highlighting the major findings from earlier reports (1-IO)@ the author has also attempted to present a synthesis of knowledge accumulated during the last 50 years. Before proceeding with this review@ the author considers it worthwhile to address the question@ ""how serious is the durability problem with modern concrete structures?"" In presenting his summary of the report of the National Materials Advisory Board's committee on concrete durability@ Skalny (1 1) questioned whether we are losing the battle for concrete durability. He made a reference to the epidemic of bridge deck deterioration in the United States. Some 253@000 bridge decks are in varying states of deterioration and approximately 35@000 are being added to this list every year. The cost of bridge deck repair and rehabilitation is $50 billion@ and the overall cost of repairing or replacing all deteriorated concrete structures is estimated to be about $200 billion. The title of the NMAB Report@ ""Concrete Durability - A Multi-Billion Dollar Opportunity@"" seems to provide an answer to Skalny's question. Concrete durability specialists in Europe seems to confirm the U.S. experience. For instance@ O'Brien et al (12) of Arup Research and Development@ London@ said@ ""Concrete durability is a growth industry: this is not to say that there is a great increase in the volume of durable concrete being produced. It is more the increase in the number of people devoted to detailed research together with contracts dedicated to repair@ refurbishment or replacement."" Reports from other parts of the world are equally disturbing. For example@ Seabrook (13) reported that in 1987-88@ on the eastern seaboard of Canada 400 of the ten-year old concrete piles of the Rodney Terminal in St. John@ which suffered cracking and spalling from freezing-thawing cycles@ had to be repaired by epoxy injection. Gerwick (14)@ in a paper on future opportunities for concrete structures in the marine environment@ said@ ""Tremendous strides have been made in the understanding of durability in corrosive environments@ yet it still remains the foremost problem facing structural concrete used today. We have only to look at the new Florida keys bridge or at many other recent undersea tunnels to see that we don't yet have foolproof answers. Indeed at one time I thought of titling this talk--Tunnels in Trouble. The Kanmon tunnels between Honshu and Kyushu respectively leak so badly that it affects not only the concrete tunnel liners but the rails and the running equipment. The Hong Kong tunnels were designed for a 120 year life and the concrete liners are in major disruption after only 4 to 10 years. The concrete in the Dubai tunnel (built in 1975) leaked so badly that the tunnel had to be completely repaired at a cost twice that of original construction. The Suez tunnel reports similar difficulties ... The new dry dock at Abu Dhabi is reportedly also severely damaged."" It should be emphasized here that around the world there are many 50- 90 year old concrete structures still in good condition. On the other hand@ numerous cases of deterioration in recently built structure go unreporfed. Therefore@ the above examples of concrete deterioration are not isolated examples. That is why a feeling of uneasiness seems to be growing among the users that concrete as a material of construction is probably not as durable as claimed by its enthusiastic advocates. For instance@ in his state-of-the-art report at the 1987 International Symposium on Concrete Durability@ Neville (15) said@ ""This symposium is concerned with concrete durability@ and it is tempting to ask why after all these years of research there are still so many problems with the durability of concrete. Possibly@ there are even more problems these days than@ say@ fifty years ago."" Lack of durability manifests in the form of cracking@ spalling@ loss of strength@ or loss of mass. Many physical and chemical causes can be responsible for deterioration@ and mechanisms and control of these causes are well-described in concrete textbooks (16-19). A concrete in an advanced state of deterioration is generally found to be suffering from more than one cause@ and in the postmortem it usually becomes very difficult to identify the first cause which might have broken down the ability of concrete to resist attack from a host of other causes. However@ failure analysis reports from around the world show that@ in order of decreasing importance@ the following causes of deterioration need most attention: corrosion of reinforcing steel@ frost action in cold climates@ chemical effects on hydrated cement paste from external agents (viz.@ water containing carbon dioxide@ sulfates or chlorides)@ and physical-chemical effects from internal phenomenon@ such as alkali-aggregate reaction@ and salt weathering. A comprehensive review of the primary causes of concrete deterioration@ which is presented next@ is therefore not in order of their importance."