Devoted to the functional morphology of endocardial endothelium (EE), this text reviews the embryological development and comparative morphology of the EE and discusses EE cell structure. To investigate the structure of EE the authors took advantage of the en face optical sectioning capacity of the confocal scanning laser microscope, which enabled them to elucidate EE cell shape, the organization of its cytoskeleton and differences compared to vascular endothelium. Permeability properties, distribution of gap junctions, von Willebrand factors, and structure of the Golgi complex are also discussed.