The electron microscope has proved to be a useful tool to study and understand the biology of platelets and to classify many platelet disorders. After a technical overview, this article reviews syndromes originating from platelet organelle, cytoskeleton, and membrane defects for which electron microscopy plays a role in the diagnostic process, such as gray platelet syndrome, Paris-Trousseau syndrome, storage pool diseases, MYH9-related thrombocytopenias, or Wiskott-Aldrich syndrome. Particular focus is given to the ultrastructural aspect of platelets in these disorders.