Identifying structural parameters can be formulated as a frequency- or time-domain optimization process, aiming to minimize the discrepancy between the measured and computed structural response time histories as described in Chapters 4 and 5. Measured or prior knowledge about the input forces is required for both approaches. For example, the identification of the railway bridge stiffness in Chapter 5 is based on prior knowledge about the trainloads such as the car length and mass and the train speed. For a majority of structures, however, the input excitation is unknown and highly difficult, if not impossible, to measure.