In sickle cell disease, a single base pair substitution in the gene encoding the beta-globin chain of the haemoglobin molecule gives rise to a surprisingly broad spectrum of pathophysiological and clinical manifestations. Inflammation, endothelial activation, red blood cell membrane abnormalities and altered availability of vasoactive factors characterise this disorder. Clinically, patients suffer from a host of seemingly unrelated maladies, from pain episodes to strokes, life-threatening infections and pulmonary hypertension. Deepened understanding of this complex disease now allows us to begin to turn away from simple supportive treatments, and move towards therapies aimed at specific pathophysiological targets. This article, the first of two reviews on the pathophysiology of haemoglobinopathies, discusses the molecular basis of sickle cell disease, and elaborates on the many factors that exacerbate or ameliorate the disease process. It then focuses on the promising targeted therapies currently in use or under investigation. An accompanying article on haemoglobinopathies (Part II) focuses on thalassaemias.