Hemophagocytic lymphohistiocytosis (HLH) is a life-threatening immune dysregulation disease. It can be subdivided into a primary, genetic form and a secondary form that complicates diverse infections, autoimmune or autoinflammatory disorders, and malignancies. Both forms, primary and secondary HLH, present with the same spectrum of non-specific symptoms, making accurate diagnosis and rapid treatment initiation challenging. A prompt diagnosis is crucial for appropriate therapeutic management. Clinical diagnosis of an HLH episode, followed by sequential immunological testing, and genetic diagnosis provide the basis for a rapid decision on hematopoietic stem cell transplantation indicated in patients with primary HLH.