White adipose tissue is a dynamic endocrine organ able to produce and release several bioactive adipokines, among them apelin, known as the ligand of the G-protein-coupled receptor APJ. The apelin/APJ system, distributed in diverse periphery organ tissues, is mainly involved in insulin sensitivity, food intake, and in the pathomechanism of childhood obesity. It regulates cardiovascular development and function, blood pressure, cardiac contractility, angiogenesis glyco- and fat metabolism and cell proliferation, apoptosis, and inflammation. Apelin inhibits water intake and vasopressin production. Apelin is mainly expressed in the endothelium, vascular smooth muscle cells, brain, heart, lung, testis, ovary, liver, placental tissue, mammary gland, and breast milk. This hormone is a key player in haemostasis with an ability to inhibit thrombin- and collagen-mediated platelet activation. The hepatic apelin/APJ system is markedly activated in progression of biliary atresia, especially in end-stage cirrhosis. In chronic heart failure plasma apelin concentrations are decreased and in patients with hypothyroidism are abnormally high. Apelin is a potential diagnostic marker for differentiating moyamoya disease and intracranial atherosclerosis and a novel biomarker for predicting type 2 diabetes in men. Children with primary hypertension show higher apelin concentration than healthy children. Apelin is implicated in the pathogenesis of childhood atopic asthma. In infants large for gestational age hiperapelinemia is noted. Apelin may function as a mitogenic agent for osteoblasts, may be involved in the development of diabetic nephropathy, retinal neovascularisation of retinopathy of prematurity, and have a diagnostic value in neonatal sepsis.