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This paper briefly summarises the process for seismic risk assessment of a single lifeline with reference to different levels of methodological complexity for hazard, exposure, fragility and performance. It goes on to describe two paradigms for modelling interdependencies between lifelines: integrated modelling which is resource-intensive since interdependencies are analysed at element-level at the precise interfaces between systems, and coupled modelling which simplifies the problem by analysing interdependencies at system-level based on measures of the strength of the interaction between the systems. The paper proposes two interdependency parameters, the coupling strength and associated adjustment factor, that can be used quantify the strength of interaction in coupled models and then presents a hierarchy for modelling interdependencies. The hierarchy consists of integrated modelling at the highest level and then four applications of coupled modelling, based on increasingly simplified methods for estimating the interdependency parameters.