This study assesses the applicability of lean for humanitarian aid organizations and what of the lean methodology is useful for humanitarian organizations. A 15 billion USD shortfall in meeting all humanitarian needs motivates humanitarian aid non-governmental organizations (NGO) to improve their efficiency and performance. Lean can reduce the impact of this financial gap and enable humanitarian NGOs to relieve more suffering. An assessment based on existing literature and a lean expert`s experience investigates how the differences in context between business and humanitarian NGOs affect the applicability of lean for humanitarian work. It shows that humanitarian aid organizations face more challenging obstacles than business organizations to become lean due to the contextual work differences. Nevertheless, this work asserts that there is an urgent need for lean in the humanitarian aid sector because it provides tools and an organizational culture that can achieve more with the limited funds they receive. The assessment of the applicability of lean tools to humanitarian operations shows that most lean tools are applicable for humanitarian operations. The lean organizational culture can benefit humanitarian organizations because lean thinking is an organizational mindset of continuous improvement which is not specific to any work context. This thesis also reviews the first organization-wide lean implementation at Medair, an international humanitarian NGO. This case study shows how lean can be adopted to the context and how this lean transformation is being rolled out in a humanitarian NGO.