Persons with Profound Intellectual and Multiple Disabilities (PIMD) are supported by professional caregivers for a large part of their lives. However, the perspectives of professional caregivers on good or poor Quality of Live (QoL) of persons with PIMD have hardly been explored. To explore the perspective of professional caregivers on QoL of persons with PIMD, we performed a qualitative study, conducting semi-structured interviews with eleven professional caregivers and thematically analysed them. In the interviews, these professional caregivers described examples of good and poor QoL. Good QoL was mostly described in emotional terms such as happiness, pleasure and enjoying things. Poor QoL was mostly described in physical terms such as pain, sickness and shortage of breath. The capability of persons with PIMD to influence the environment was described as an element of good QoL. Furthermore, these professional caregivers described how they assessed QoL. They 'sensed' QoL, they tested it (trial and error) and discussed QoL of persons with PIMD with colleagues. Participants emphasized the dependency of persons with PIMD on professional caregivers. Furthermore, these professional caregivers described feelings of failure and powerlessness if they could not assess or fulfil the needs of persons with PIMD. We conclude that these professional caregivers use their (tacit) knowledge to understand QoL of persons with PIMD. The relationship between QoL of persons with PIMD and professional caregivers needs more investigation, because of its consequences both for the persons with PIMD and the professional caregivers.