The United States is a comparatively young country and many of structures are relatively new. About 80 percent of the existing construction was built in the last 75 years. Much of this is becoming obsolete. Yet little is being done to change our concept of building. Our practice is to build, tear down, then build up again- a never ending process. Frequently, we are not even sure when replacement is structurally necessary. How does one load test a structure? Decisions to tear down and rebuild are being made daily. What are the considerations that lead to these decisions, and how many of them are valid? A small number of structures come to an early demise from unexpected failures caused by defective design, in adequate or defective inspection, in adequate workmanship or some combination of these. When design has been inadequate it may be because hitherto unknown or unappreciated environmental factors were at work. This happened when the alkali-aggregate reaction was first discovered. Or design may have been inadequate because the environment became more hostile at some time after the structure came into use. Although the subject of this study is the life span of structures another concept might also be considered, the half-life of the structure. New requirements are continually being placed on any building. Unless it can meet these new requirements or be readily changed to meet them it arrives at its half-life and maintenance is discontinued. Thus good building design requires that it be possible for the structure to be upgraded for it to evolve to meet new needs. How can a building be made to evolve? One way is to forecast the kind of space that may be needed in the building during the coming period-say 50 years, to design in a manner that will permit subdividing the space in any of many ways, and then to change the subdivisions as needs change. If its design is sufficiently flexible a building can go through many uses before it becomes obsolete.