Background: Recognizing the need for palliative care (PC) and referral to PC teams improves patients' quality of life. However, in patients with moderate/severe clinical complexity, early recognition of the need for PC may not correspond to referral to specialized PC services. The definition for clinical complexity is still underexplored, as well as the instruments available to assess complexity. This scoping review aims to gather relevant information on the definition of clinical complexity in PC, as well as on the instruments used to objectively assess complexity.Methods: According to the methodology of a Scoping Review, the keywords: "palliative care", "hospitalization criteria", "complexity criteria", "complexity assessment" and "clinical complexity", were searched in PubMed, Scopus, Cochrane, and b-on databases, during April 2022, for relevant information on the definition and/or approach and/or protocols related to clinical complexity in patients followed in PC, or on the instruments used to assess it, regardless of study design, the language, or year of publication.Results: From the 626 references found, 15 studies were included in the review. According to these studies, complexity may be organized/defined into 3, 4, or 6 domains, generally including the patient, the family, the health system, and the socio-cultural context. Of the 13 instruments mentioned for the objective assessment of complexity, the HexCom, IDC-Pal, and the recent ID-PALL seem to offer the broadest determinations of complexity.Conclusions: Complexity is a dynamic process, which reflects the reality of patients and families, and patients, families, and health professionals' perceptions, and so it must be systematically adjusted to the stage of the disease. The definition of complexity and the development and use of suitable instruments can help to identify, assess, and improve patients' quality of life, while supporting their family across the grieving process.Yet, this may not always be summarized in a quantitative value by easy-to-use instruments, highlighting the role of PC interdisciplinary teams.