This chapter describes how a confocal microscope can be treated as a spectrophotometer to measure the absolute number of fluorescent molecules in live cells (Wu and Pollard, 2005).(1) The method provides dynamic range of over three orders of magnitude for counting the number of molecules in a single cell. We present a step-by-step guide to measure concentrations in vivo, explaining many of the practical considerations for using this technique. This chapter is meant as a resource for cell biologists, biochemists, and biophysicists interested in quantifying macromolecules involved in their favorite molecular pathways in live cells.