The characterisation of strong ground motion is a crucial element of analyses of seismic risk for isolated structures and lifeline networks. Many measures of the strength of ground motion (intensity measures) have been developed. Each intensity measure may describe different characteristics of the motion, some of which may be more adverse for the structure or system under consideration. The use of a particular intensity measure (IM) in seismic risk analysis should be guided by the extent to which the measure corresponds to damage to local elements of a system or the global system itself. Optimum intensity measures are defined in terms of effectiveness, efficiency, sufficiency, robustness and computability. A detailed review of studies into the optimality of different intensity measures for specific structural systems shows that the most efficient intensity measures are those that relate directly to the structure under consideration. For each system considered in the lifeline analysis, preliminary recommendations are made as to the most appropriate IM to use in fragility and vulnerability models. These recommendations are guided on the basis of current state-of-the-art in the relevant fragility models, in addition to consideration of the physical response of elements within the system. For buildings, building aggregates and structures, most current fragility models are defined in terms of spectral acceleration at the fundamental period of the structure (Sa[T0]), whilst non-structural and/or mechanical elements are more sensitive to peak ground acceleration (PGA). Within a lifeline analysis, however, structural failure may also occur due permanent ground deformation (e.g. slope displacements, liquefaction etc.), which may also correlate to energy and duration-based IMs. Definition of the strong motion input for system analysis will likely need to consider multiple IMs in order to implement vulnerability analyses in the most efficient manner.