This cluster of papers focuses on the diagnostic approach to fever of unknown origin (FUO), with a particular emphasis on the use of FDG PET/CT imaging, bone marrow biopsy, and the evaluation of pancytopenia. It encompasses studies on the clinical evaluation, etiological profile, and diagnostic algorithms for identifying the cause of FUO, often related to inflammation and hematological malignancies. The cluster includes numerous prospective studies, meta-analyses, and systematic reviews highlighting the importance of these diagnostic tools in managing patients with FUO.