Abstract: Nondestructive testing (NDT) for flaws in materials and structures has undergone an evolutionary change over the past 50 years. In the U.S. it has moved from a testing strategy (NDT) with a zero defects requirement to a test and evaluate procedure (NDE) based upon damage tolerant design considerations. Here it is assumed that the part will always contain defects but those greater than a critical size, specified by fracture mechanics, will be removed by inspection thereby resetting the parts service clock. In this talk, events will be identified that were critical in promoting this paradigm shift and in the development of a quantitative NDE (QNDE) technology. A number of major research programs were initiated to upgrade NDT to meet the new requirements; principal attention in this talk will be given to research highlights begun in the DARPA/AFML Interdisciplinary Program for Quantitative Flaw Definition that was initiated by the author. Its purpose was threefold: to develop a new core science/people base for inspection technology that could meet the new requirements, to set the stage for new field - adaptable engineering tools, and to initiate a continuing series of quantitative NDE (QNDE) research meetings. Advances initiated in this program and pursued by many over the years have resulted in a scientific core structure for quantitative NDE (QNDE) based on a linkage of fundamental models of the various measurement processes that are involved in any inspection and/or technology. These models and their linkage will be discussed and the core structure defined. A new and powerful set of engineering tools - i.e. simulation programs for UT, X-ray, and EC technologies–has also been developed using these models. Applications of these tools will be highlighted and their role in other advanced programs including Structural Health Monitoring and Condition-Based Maintenance will be noted. Finally, a discussion of visions of future opportunities and directions for QNDE will be given.