The role of the pressure exciting a mode of vibration of a structure may be expressed in terms of the generalized force, which involves the two-point space-time pressure correlation over the exposed surface of the structure. This is illustrated in the one-dimensional case, the significance of scale being pointed out. The general two-point correlation is needed for other cases, except in the important special case of the generalized force for plane waves exciting orthogonal modes of a rectangular surface when the product of instantaneous correlations taken parallel to the edge suffices. Response of structures large as compared to the scale of the pressure field may be found approximately from a corresponding infinite structure, regardless of the magnitude of the structural damping, provided the few lowest modes are not highly excited. When more than a single mode is excited, the correlation between the motions of the modes may become important, especially in determining maxima of the response: this is particularly so for cylindrical and spherical structures. The adequate determination of the pressure correlation in many practical situations is very laborious; also, there is often a lack of detailed knowledge of the relevant structural-response characteristics: model test work may be most useful, provided that the ratio of excitation to natural frequencies, and of excitation scale to structural scale, is preserved. If nonlinear effects are present in either the presure field or in the structure, the amplitude must also be appropriately scaled.