The advent of deep drilling has served to emphasize that the design of casing strings is an engineering problem. For this reason it is essential that minimum performance characteristics of casing be established and used, rather than the average properties. It is only by this procedure that confidence can be placed in the factors of safety computed. The test data were analyzed critically, and minimum performance properties of casing calculated which are proposed for use until additional test data permit the establishment of higher permanent minimum values. The necessity for basic revision of the API pipe specifications to meet present and future needs of the industry is pointed out. Factors to be considered in the design of casing strings for deep wells are discussed in as much detail as present knowledge warrants.