Biomarkers are biological indicators of processes that are part of ethiopathogenesis of the diseases, and can, but do not have to be causal to diseases. One very important question is how specific and sensitive the marker is, since one molecule can appear in many conditions. Biomarkers of endothelial cell activation can be very diverse, from biochemical/ metabolic to functional biomarkers. Activation of endothelial cells is part of physiological as well as pathophysiological response of cardiovascular system in conditions as physical activity, growth, pregnancy and in all cardiometabolic diseases (e.g., hypertension, diabetes mellitus, autoimmune inflammatory diseases, coronary artery disease, atherosclerosis, ischemia and reperfusion, etc.). During activation, there is a change in endothelial cell morphology and function, which could be a defensive response of endothelium to provoking factor or could lead to increased risk for the injury and end organ damage. This chapter aims to overview current knowledge on established biomarkers of normal and disease-related endothelial activation and to provide information on novel, potential biomarkers in common cardiometabolic diseases.