Complex systems being built and in use today cannot be adequately analysed by using “traditional” tools and techniques, since, among other properties, they exhibit emergent behaviours (i.e. behaviours that cannot be explained by examining the system’s individual components). This guidance describes tools and techniques that can either replace or augment traditional ones to take better account of a system’s complexity throughout its lifecycle. Appendix A on page 45 lists some of the inadequacies of traditional techniques. This guidance has been written in this context: how can we justify the safe operation of a complex system given that the traditional assumptions are no longer valid?