Photosynthesis is one of the most readily measured metabolic processes in a plant, with fluxes being measurable non-invasively even under field conditions. In this chapter, two principal approaches are described to measure photosynthesis--O2 evolution as determined using an O2 electrode, and CO2 fixation which can be quantified using an infrared gas analyser. The advantages and disadvantages of these different methods, as applied to Arabidopsis, are discussed, and some major forms of analysis are described.