There are numerous emergencies that can arise in hematological patients. Patients with malignant and non-malignant hematological diseases can have dramatic, often life-threatening clinical presentations. Although these disorders are mainly uncommon, it is necessary to recognize a hematological emergency in a timely manner, as well as to know how to initiate basic treatment. Hematological emergencies have frequently been neglected in internal medicine textbooks, or not presented as such, and therefore described in various chapters. In articles concerning hematological emergencies, it is common that different authors describe completely different sets of hematological emergencies. However, that is understandable, since it concerns a large number of disorders with various etiopathogeneses, both malignant and non-malignant, many of which remain unresolved, and therefore present a diagnostic and therapeutic problem. Many hematological emergencies are complications of primarily hematological processes, whereas others are entities which can develop in patients having diseases of some other etiology. This clinical review describes pathophysiology, clinical presentation, diagnostics and treatment of some of the most common hematological emergencies (e.g. hypercalcemia), as well as of some of the infrequent ones (e.g.TTP-HUS), whose importance lies in difficult diagnostics, incidence in pediatric and young adult age and potentially lethal outcome.