Oxygen belongs to one of the most important analytes on Earth. Oxygen sensing via phosphorescence quenching offers many advantages over other methods. In the last decades it became the method of choice for numerous applications in research and industry. The choice of appropriate material is particularly important for successful realization of these applications. This chapter focuses on the fundamentals of oxygen detection with phosphorescent sensors and discusses general principles of materials design including the selection of suitable indicators and matrices, adjustment of the dynamic range of the oxygen sensors, referenced sensing and imaging as well as possible measurement artefacts.