Over the past of 50 years, my research group, together with collaborators at Rice University and other institutions in the United States, the United Kingdom, Italy, Germany, Belgium, and Japan have examined the mechanisms for O₂ storage and transport by hemoglobins and red blood cells on distance scales from millimeters to tenths of nanometers and timescales from seconds to picoseconds. The results from these biophysical and structural biology studies have provided a detailed framework for: (i) examining the evolution of hemoglobins in both plants and animals; (ii) understanding the efficiency of O₂ transport by red blood cells in capillaries and O₂ storage by myoglobins in muscles; (iii) determining the toxicity of free hemoglobin in plasma after red blood cell lysis; and (iv) developing heme protein-based gas sensors and blood substitutes for industrial and clinical applications.