In practice are often observed patients with thrombocytopenia requiring the correction by transfusion of platelet. However, the cause of the platelet's reduction is not always obvious, what needs to be clarified. In this article, we described the main pathogenic types of thrombocytopenia. We present an algorithm that will help the physicians to understand the causes of thrombocytopenia and to determine the necessity of platelet transfusions depending on the clinical settings and on the type of thrombocytopenia. Absolute indications for platelet transfusions are surgeries and life-threatening bleeding, while the relative indications are thrombocytopenia due to consumption.