During the last decade, infrared spectroscopic techniques have entered the field of photosynthesis and they are meanwhile established methods for probing chlorophyll or quinone molecules in their binding sites and for the investigation of molecular processes in the protein upon electron transfer or proton transfer. This advancement has mainly become possible with the rapid development and availability of Fourier transform infrared (FTIR) spectrophotometers, but also with the development of sensitive infrared semiconducor detectors and tunable IR lasers or picosecond IR laser systems. This chapter introduces the basic concepts for using IR Spectroscopy to study such complex systems as a reaction center or even complete photosynthetic membranes. The basic principles of FTIR spectroscopy for steady-state or time-resolved investigations will be described, and examples from the study of primary electron transfer in reaction centers will be given. Single-wavelength time-resolved techniques for the millisecond-to-nanosecond and pump-probe techniques for the picosecond time domain are reported. A separate section deals with the strategies for the assignment of IR bands, an essential issue for the use of IR techniques in studies of macromolecules. Furthermore, techniques for the preparation of IR samples from photosynthetic membranes or reaction centers are described.