Patients hospitalized for an acute illness or injury are at risk of experiencing a significant loss of functioning as defined by the International Classification of Functioning, Disability and Health (ICF). The risk of a significant loss of functioning is increased in critically ill patients, in patients with complications or long-term intensive care stays, in persons with disabilities or with pre-existing chronic conditions and in the elderly. Early identification of rehabilitation needs and early start of rehabilitation can reduce healthcare costs by reducing dependence and nursing care, length of stay and prevention of disability. Two principles of rehabilitation for acute and early post-acute care can be distinguished. First, the provision of rehabilitation by health professionals who are generally not specialized in rehabilitation in the acute hospital. And second, specialized rehabilitation care provided by an interdisciplinary team. There is large variation how this specialized, typically post-acute rehabilitation care is organized, provided, and reimbursed in different countries, regions, and settings. For instance, it may be provided either in the acute hospital or in a rehabilitation or nursing setting. Most in-patients do not receive specialized rehabilitation at all during their whole stay in the acute hospital. But, it is important to point out that health professionals working in acute hospitals and who are not specialized in rehabilitation need to be able to recognize patients' needs for rehabilitation care and to perform rehabilitation interventions themselves or to assign patients to appropriate rehabilitation care settings. The principles outlined in this paper can serve as a basis for the development of clinical assessment instruments to describe and classify functioning, health and disability of patients receiving acute or early post-acute rehabilitation care.