Haemoglobin carries oxygen from the environment to tissues; in vertebrates, it is contained in specialized cells, called erythrocytes. Over the last century, the study of the chemical properties of this haemoprotein has provided a wealth of information. One of its most important and ancient physiological features is the Root effect, found in many teleost fish (and some amphibians). The Root effect corresponds to an extreme pH sensitivity and can be described as an exaggerated Bohr effect: it dictates to what extent the oxygen tension can be raised in acid-producing tissues. It is likely that the eye choroid rete represents the most ancient anatomical structure associated with the presence of Root effect haemoglobins. This review describes our overall understanding of the molecular properties, biological occurrence, physiological role and evolutionary origin of Root effect haemoglobins. The current knowledge of the structural properties of Root effect haemoglobins is discussed in the light of recent results obtained on the haemoglobins of the coldadapted notothenioids Trematomus newnesi and T. bernacchii.