Failures of critical infrastructures can represent a threat both to people, economy and societal functions and to national security. So, thorough risk analyses of infrastructures are required to reduce the probability and mitigate the consequences of failures. The interdependencies between infrastructures can be strong, but are seldom accounted for in current analyses. This chapter presents a method for assessing these interdependencies and also provides an example. The analysis is part of an overall cross-sector risk and vulnerability analysis (RVA), see Chap. 3.