On-line NDE practices typically emerge through a sequence of steps starting with the selection of a particular technique, proceeding with definition of configurations and procedures, and culminating in qualification through an experimental demonstration of performance. Because of the absence of a well-developed analytical foundation, this process has been, by necessity, highly empirical in nature. The theoretical advances in the understanding of field-flaw interactions which have occurred over the last decade provide the tools necessary to adopt a more analytical approach to the engineering of a new NDE system. This paper explores the present status of these tools and discusses different ways in which they have been, or might be, used to quantify and improve inspection reliability.