Fluorescence correlation spectroscopy (FCS) performed using a laser scanning confocal microscope is a technique with single-molecule sensitivity that is becoming more accessible to cell biologists. In this chapter, we describe the use of FCS for the analysis of diffusion coefficients and receptor-receptor interactions in live cells in culture. In particular, we describe a protocol to collect fluorescence fluctuation data from fluorescence-tagged receptors as they diffuse into an out of a small laser-illuminated observation volume using a commercially available system such as the Zeiss ConfoCor 3 or LSM-780 microscope. Autocorrelation analysis of the fluctuations in fluorescence intensity provides information about the diffusion time and number of fluorescent molecules in the observation volume. A photon-counting histogram can be used to examine the relationship between fluorescence intensity and the number of fluorescent molecules to estimate the average molecular brightness of the sample. Since molecular brightness is directly proportional to the number of fluorescent molecules, it can be used to monitor receptor-receptor interactions and to decode the number of receptor monomers present in an oligomeric complex.