PURPOSE: In the legal performance of the euthanasia procedure, unbearable suffering, one of the requirements of due care, is difficult to assess. Evaluation of the current knowledge of unbearable suffering is needed in the ongoing debate about the conditions on which EAS can be approved. METHODS: Using an integrative literature review, we evaluated publications with definitions of suffering in general or in end-of-life situations and with descriptions of suffering in the context of a request for EAS. DATA SYNTHESIS: From the 1482 citations identified, we included 55 publications: 20 articles about definitions of suffering and 35 empirical studies on suffering. We found no definition of unbearable suffering in the context of a request for EAS. Qualitative patient-centered studies revealed the most motivations, and the most motivations named by only one of the three parties involved. The studies of relatives were limited, mainly quantitative and retrospective. We found no studies that brought together the views of the patients, relatives, and healthcare professionals. CONCLUSIONS: There is no generally accepted definition of 'unbearable suffering' in the context of a request for EAS. On the basis of the articles reviewed, we propose the following conceptual definition: 'Unbearable suffering in the context of a request for EAS is a profoundly personal experience of an actual or perceived impending threat to the integrity or life of the person, which has a significant duration and a central place in the person's mind'. Further patient-centered qualitative research into suffering is needed to clarify this definition.