O(2)carriers (extracellular and intracellular as well as monomeric and multimeric) have evolved over the last billion of years, displaying iron and copper reactive centers; very different O(2)carriers may co-exist in the same organism. Circulating O(2)carriers, faced to the external environment, are responsible for maintaining an adequate delivery of O(2)to tissues and organs almost independently of the environmental O(2)partial pressure. Then, intracellular globins facilitate O(2)transfer to mitochondria sustaining cellular respiration. Here, molecular aspects of multiple strategies evolved for O(2)transport and delivery are examined, from the simplest myoglobin to the most complex giant O(2)carriers and the red blood cell, mostly focusing on the aspects which have been mainly addressed by the so called 'Rome Group'.