Dynamic molecular interactions are fundamental to all cellular processes. In vivo analyses of these interactions are frequently done using fluorescence recovery after photobleaching (FRAP). Proper interpretation of FRAP data yields information about the binding interactions of fluorescently tagged molecules, including the number of binding states and the binding strength of each state. This binding information can be gleaned from appropriate models of the process underlying a FRAP recovery. Continued application and development of these approaches promise to provide crucial information for a quantitative description of the molecular networks that regulate cellular function.