This review outlines key NGO perspectives on Level 3 (L3) crisis designations by providing an overview of how national and international Non-Governmental Organisations (NGOs) have understood and responded to four of the five declared L3 crises to date. Building on the Humanitarian Reform process, in December 2011 the Inter Agency Standing Committee (IASC) launched the Transformative Agenda (TA) as a set of actions that collectively represent a substantive improvement to the current humanitarian response model. Among these actions was the creation of guidance for response to L3 emergencies, major humanitarian crises which require system-wide mobilisation ‘beyond normally expected levels’. The TA’s guidance on L3 emergencies includes but is not limited to the engagement of national and international NGOs throughout the humanitarian programme cycle. This review included a series of semi structured key informant interviews with NGO staff, coordinators and other humanitarian actors who had participated in L3 responses, as well as field visits to Iraq and South Sudan, both ongoing L3 crises at the time of writing.