Signal transduction pathways underlay the communication of cells with their environment. For example, how do yeast cells faithfully find a mating partner? We study the MAP kinase signal transduction pathway that underlies signal transduction during mating. We established Fluorescence (Cross-) Correlation Spectroscopy [FCCS] and FLIM to work with yeast cells (Ref). These new quantitative imaging methods enable us to measure protein complex formation and to visualize the activity of the MAP kinases. This yields important new insights into the dynamics and the spatial organization of the signaling process.