Understanding diamagnetism of oxygenated and paramagnetism of deoxygenated hemoglobin, an important topic in (bio) chemistry for medical students, begins with the understanding of the detailed structure of the oxygen molecule. It continues with a short description of the structures of reactive oxygen species: the singlet form of oxygen, the superoxide, and peroxide anions. The article then describes the high spin and low spin form of the iron ion with its d-orbitals and explains why both forms with different diameters exist in the heme part. At the end, medical applications of the acquired knowledge are presented: the cooperative effect in hemoglobin and the principles of magnetic resonance imagining.