The chemistry of singlet molecular oxygen [1O2 (1Delta g)], its importance in atmospheric, biological, and therapeutic processes, and its use as a reagent in organic synthesis have been of considerable interest. Many aspects of singlet oxygen chemistry have emanated from the work of Christopher S. Foote and co-workers. Singlet oxygen is a historically interesting molecule with an unusual story connected with its discovery. Foote and Wexler conducted experiments in the 1960s where evidence was obtained supporting 1O2 generation via two independent routes: (1) a photochemical reaction (dye-sensitized photooxidation) and (2) a chemical reaction (NaOCl with H2O2). An important factor in the discovery of 1O2 as the critical reaction intermediate in dye-sensitized photooxygenations was Foote's reassessment of the chemical literature of the 1930s, when 1O2 was suggested to be a viable intermediate in dye-sensitized photooxidation reactions. Experiments that used silica gel beads provided evidence for a volatile diffusible oxidant such as 1O2. However, a contemporaneous quarrel surrounded this early work, and the possible existence of solution-phase 1O2 was ignored for over 2 decades. Not long after Foote's initial studies were published in 1964, the idea of singlet oxygen as an intermediate in photooxidation chemistry gained increasing recognition and verification in organic, gas phase, and biological processes. There are many documented impacts that 1O2 has had and continues to have on biology and medicine, for example, photodynamic therapy and plant defenses.