The dynamical behaviour of civil engineering structures has traditionally been tackled, for design purposes, in an ‘equivalent static’ way, essentially by introducing magnification factors for vertically applied loads and/or by specifying equivalent horizontal loads. Today the availability of software able to deal explicitly with dynamic analysis of realistic structures with many (dynamic) degrees of freedom, as well as the outcome of the valuable research carried out in the various fields included under ‘Dynamics’, make this type of analysis a part of everyday life in the design office. There are also a number of good reasons why dynamical behaviour of buildings, bridges and other structures is now more of a concern for the designer than it used to be 20 or 30 years ago. One reason is that the aforementioned structures currently consist of structural members that are more slender than before, and lighter cladding made of metal and glass or composites rather than of brick walls.