Shear-wave velocity structure is a crucial factor for site characterization and site effect assessment. There are several methods for estimating velocity structure (intrusive or nondestructive, with active or passive sources). We are not going to compare these different methods, but we will point out that all methods have certain limits: penetration depth, resolution, and cost are the main limiting factors. For instance, acquiring a complete and reliable velocity profile from surface to hard bedrock is often difficult and expensive. It is obvious that an alternative kind of parameter, instead of the complete velocity profile, should be used for site characterization.