Providing water, sanitation, and hygiene (WASH) to emergency-affected populations is necessary for reasons of dignity and disease control. Such a (humanitarian) response is coordinated via the 'cluster approach'. This study utilises a literature review, an appraisal of and analysis of Global WASH Cluster (GWC) documentation, and key informant interviews to summarise the outcomes and impacts of GWC coordination. Across these three datasets, consistent themes were identified, including: the cluster approach evolving into a cost-effective 'best-fit' model; cluster staff requiring technical and coordination skills; and cluster members facing participationrelated trade-offs. Consistent intractable difficulties were pinpointed, too, such as: accountability to beneficiaries; cross-cluster and subnational cluster coordination; and working with national governments. Previous research was found to be largely subjective and not to address future cluster challenges. An analysis of cluster outcomes and impacts, including member and beneficiary perspectives, is needed. To facilitate this work, a theory of change for cluster coordination was also developed.