Within the last 100 years the role of platelets in hemostatic events and their production by megakaryocytes (MKs) has been gradually defined. Progressively, thrombocytopenia was recognized as a cause of bleeding, first through an acquired immune disorder; then from 1948 when the Bernard-Soulier syndrome was first described, inherited thrombocytopenia has become a fascinating source of Mendelian disease. Often the platelet count is severely decreased and platelet size variable; associated platelet function defects often aggravate bleeding. Macrothrombocytopenia (MTP) with enlarged platelets in variable proportions is common. The circulating count will depend on platelet production, consumption and lifespan. The bulk of MTPs arise from defects in megakaryopoiesis with causal variants in transcription factor genes giving rise to altered stem cell differentiation and changes in early MK development and maturation. Genes encoding surface receptors, cytoskeletal and signaling proteins also feature prominently and Sanger sequencing associated with careful phenotyping allowed their early classification. It quickly become apparent that many inherited thrombocytopenias are syndromic while others are linked to an increased risk of hematological malignancies. The application the last 10 years of Next Generation Sequencing (NGS) including whole exome sequencing and the use of gene platforms for rapid testing has greatly accelerated the number of causal genes as well as extending the list of variants in more common disorders. Genes linked to an increased platelet turnover and apoptosis have also been identified. The current challenges are now to use NGS in first step screening and to better define bleeding risk and treatment.