In recent years, single molecule spectroscopy has provided novel insights into the fundamentals of electronic excitation energy transfer in molecular aggregates. In order of increasing structural complexity, we have studied simple molecular dimers and multichromophoric dendrimers. It will be shown that the combination of frequency-selective single molecule spectroscopy and confocal fluorescence microscopy at 1.4 K is a unique tool to study energy transfer processes in these systems. In particular, from the line widths of single molecule excitation spectra, rate constants of energy transfer can be deduced directly. A detailed analysis shows that, for several cases, the mechanism of energy transfer cannot be described by the simple Förster model (dipole–dipole coupling). Furthermore, novel experiments with dual color pulsed excitation are discussed which allow for controlling the excitation energy flow in individual molecular dimers.