The development of a complex intervention inevitably requires the preparation of a systematic review of the existing evidence in order to inform all steps of the development and evaluation processes. Beyond the general methodological challenges of systematic reviews and meta-analyses, systematic reviews of complex interventions pose specific challenges. In particular, the reporting of primary studies of complex interventions is often not sufficient to produce meaningful summaries of the research evidence (Guise et al., 2014). As a consequence, systematic reviews of complex interventions regularly provide insufficient information. Frequently, randomized controlled trials of complex interventions that have been developed and evaluated carefully through different development phases are combined with trials of insufficiently developed interventions. Studies evaluating the efficacy of an intervention for the first time are often combined with studies that transfer a programme to another context. In view of these difficulties, this chapter addresses a number of challenges encountered when preparing systematic reviews of complex interventions.