In the case of a will made by a person suffering from cancer, sometimes the party seeking to challenge it will raise as an argument the very fact that the testator has been diagnosed with cancer (regardless of its type, severity of symptoms, phase of development and dynamics). In the opinion of beneficiaries omitted in the will, this would deprive the testator of the ability to make a conscious or free decision and express his will. A substantive analysis of the court material requires that the experts not only follow the general principles of preparing psychiatric and psychological opinions in testamentary cases but also take into account certain psychopathological conditions and other factors specific to cancer. The aim of this literature review is to present these special circumstances in the context of the possibility of their use by experts when preparing an expert opinion on the assessment of the validity of the declaration of will. In assessing the mental state of the testator, experts should consider not only reactive mental disorders, cognitive deficits or disorders of consciousness, but also conditions characteristic of the neoplastic process: CRCD (cancer-related cognitive dysfunction), CRF (cancer-related fatigue), CACS (cachexia-anorexia cancer-related syndrome). In their assessments, experts cannot ignore other factors specific to this disease: its phase nature, the degree of adaptation of the testator to the disease, family conditions that may influence the testator’s property decisions.