Cancers are known to increase the tendency for thrombosis, both on the venous and arterial side, which to this day is an important factor in the management of oncology patients. Malignant disease is an independent risk factor for developing venous thromboembolism (VTE). Thromboembolic complications in addition to the disease worsen prognosis and are accompanied by significant morbidity and mortality. VTE is the second most common cause of death in cancer after disease progression. Tumors are characterized by hypercoagulability, in addition to which venous stasis and endothelial damage also occur in cancer patients promoting increased clotting. Treatment of cancer-associated thrombosis is often complex; therefore, it is important to identify patients who benefit from primary thromboprophylaxis. The importance of cancer-associated thrombosis is indisputable in everyday oncology. We briefly summarize the frequency and characteristics of their occurrence, the underlying mechanisms, risk factors, clinical appearance, laboratory diagnostics, and the possibilities of prevention and treatment.