Platelets play a central role in hemostasis, wound healing and the regulation of inflammation, angiogenesis, and immune response. In pathology, they are not only involved in hemostasis and thrombosis but also in atherosclerosis, metastasis, and viral infections. Their number varies from 1.5 to 4.5 × 105/μL and with a half-life of 8–11 days, they must be constantly renewed by a highly dynamic process called megakaryopoiesis, which produces 1011 platelets per day. Megakaryopoiesis begins with the commitment of hematopoietic stem cells (HSCs) to the megakaryocyte (MK) lineage and the proliferation of committed progenitors. These two processes are common to those of other hematopoietic lineages, although there are some similarities between the HSC and MK programs, with the description of a megakaryocytic/platelet HSC. The other two stages, specific to MK differentiation, consist of a polyploidization or endomitosis and dynamic cytoplasmic fragmentation. In adult humans, the entire process takes place mainly in the bone marrow, with the exception of platelet release, which occurs in the blood stream. Megakaryopoiesis is regulated by intrinsic factors such as transcription factors, and by extracellular signals dominated by thrombopoietin (TPO), which binds to its receptor MPL (TPO-R) and activates a downstream cascade entirely dependent on JAK2. Platelet formation is itself controlled by dynamic changes in the cytoskeleton regulated by the interaction of MK with the bone marrow environment in the vicinity of vessels. Genetic alterations at any stage of this process lead either to thrombocytopenia or thrombocytosis. However, thrombocytopenia could also be secondary to autoimmune disorders, inflammation, and infections. In this chapter, we will first focus on hereditary thrombocytopenia due to genetic alterations in TPO/MPL signaling pathway, transcription factors, proteins involved in cytoskeletal reorganization, glycosylation and sialylation, α-granule biogenesis, and metabolism. Then, we will discuss the defects of megakaryopoiesis implicated in thrombocytopenia related to autoimmune disorders such as idiopathic/immune thrombocytopenic purpura (ITP) and acquired immune amegakaryocytic thrombocytopenia (AIAT). Finally, we will address the mechanisms leading to thrombocytopenia secondary to infections.