Deflection testing and backcalculation have been conducted on all types of concrete highway and airfield pavements for many years. Backcalculation of the in situ slab elastic modulus and the effective k-value or elastic solid subgrade modulus (beneath the slab) from deflection basins was first proposed by Westergaard in 1925 and actually conducted in the 1930s. Over the past decade, backcalculation has greatly improved and is now extremely rapid, highly automated, and reliable. The resultsfrom concretepavement deflection testing and backcalculation have been used for several purposes, to compute the load-carrying capacity of concrete pavement for aircraft and truck loadings, to design structural overlays, and to detect loss of support beneath slab corners. Thus deflection testing and backcalculation of in situ concerte pavement properties have proven to be very beneficial. The theory and practice of backcalculation for concrete pavements as they have developed over the years are summarized, results from a wide variety of pavements throughout the United States are presented, and implications for evaluation and design of concrete pavements are discussed.