Following a large-scale disaster, such as a major earthquake, tsunami or cyclone, tens of thousands of persons are often displaced, suffer from food shortages and in need of medical assistance. In situations in which the State affected by the disaster does not meet the needs of the affected persons itself, humanitarian assistance from outside the State might be required. This article considers the role of consent to external humanitarian assistance on the part of the affected State. As there is no single overarching treaty in the area of humanitarian assistance in situations of disaster, the article explores the role of consent in the various disaster-specific, subject-specific and region-specific treaties as well as in the soft law instruments in the area. Although the instruments take seemingly different approaches to the subject, a common standard is identified, namely that consent on the part of the affected State is required before external assistance can be provided but that consent cannot be arbitrarily withheld. The article then goes on to give content to the arbitrary withholding standard, breaking it down into its substantive and procedural elements. These include the meaning of the term ‘arbitrary’; the requirement to provide a reason for the withholding of consent; legitimate grounds for withholding consent; and the actor that assesses the justification. Regard is had for State practice in the context of disasters as well as other areas of the law in which similar tests are used.