Mental health and psychosocial support (MHPSS) practitioners have, like other humanitarian aid providers, grappled with issues of coordination and associated problems, such as duplication of services, gaps in delivery and unmet needs. To combat this, a mapping tool has been developed to address the issues particular to MHPSS programming, both during and following an emergency. 'Who is Where, When, doing What' (4Ws) is a practical tool to assist the mapping of MHPSS interventions.