The near-IR phosphorescence of singlet delta oxygen, O2(a1Ap), has provided a wealth of information since it was first observed in solution-phase systems. The techniques employed and the quality of the data obtained have improved significantly over the years that, in turn, presently makes it possible to address a wide variety of problems using both steady-state and time-resolved measurements. The development of spectroscopic methods to monitor other transitions in oxygen, specifically those that involve the singlet sigma state, 02(b1X;), and the incorporation of high-level computational methods provides access to an even broader range of fundamental issues. The expertise presently available to monitor radiative transitions in oxygen, coupled with the current understanding of the effect of solvent on these transitions as achieved through state-of-the-art theoretical modeling makes it possible to consider the next step forward: the incorporation of spatial resolution and the construction of the singlet oxygen microscope.

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