A great deal of information about a nuclear power plant and the coefficients that describe it is contained in the data that can be collected in plant transients. Much of this information is difficult or impossible to obtain from steady-state measurements. Significant advances have been made in developing techniques to extract the desired performance-related or safety-related information from transient data records. Techniques are available for determining such specific design parameters as reactivity feedback coefficients or heat transfer coefficients. Models, either derived from physical principles or developed empirically, can be tuned by comparison with plant data, and they are capable of very accurate predictions of plant responses to disturbances. Efficient methods, with on-line computing capability, can track performance-related parameters to yield information on plant conditions for surveillance purposes. Methods such as these provide expanded capability for extracting useful information from operating plants.