Recent developments in cell biology and microscopy techniques enable us to observe macromolecular assemblies in their natural setting: the living cell. These emerging technologies have revealed novel concepts in nuclear cell biology. In order to further elucidate the biochemistry of gene expression, replication, and genome maintenance, the major challenge is now to precisely determine the dynamics of nuclear proteins in the context of the structural organization of the nucleus. Fluorescence correlation spectroscopy (FCS) is an attractive alternative to photobleaching and photoactivation techniques for the analysis of protein dynamics at single-molecule resolution. Here we describe how FCS can be applied to retrieve biophysical parameters of nuclear proteins in living cells.