Radiationless transfer of excitation energy is at the hear of many processes in quantum physics, chemistry and nanotechnology. Currently, the standard picture of an incoherent Förster resonant excitation transfer is being challenged by the experimental findings of a long-lived quantum mechanical coherence in biomolecular light harvesting complexes. The role of this in molecular aggregates is addressed in the first part of this volume. Utilizing some of the underlying principles to optimize nano scale devices, the second part addresses systems of colloid quantum dots and polymer based organic solar cells.