A consistent set of definitions is given of the principal quantities pertaining to the oxygen transport by the blood, and of their mutual relationships, in relation to the methods used in their measurement. At the core is the correct definition of oxygen saturation, the deviation of which has recently been the cause of considerable confusion, especially concerning pulse oximetry. The occurrence, properties, determination, and pathophysiological and clinical significance of dyshaemoglobins, such as carboxyhemoglobin, methemoglobin, and sulfhemoglobin, are briefly described, together with possible consequences for diagnosis and therapy. In addition, attention is payed to the use of some haemoglobin derivatives, such as methaemoglobin and cyanmethaemoglobin, for clinical chemical and therapeutic purposes.