Processes leading to fluorescence quenching of organic molecules in liquid solution were investigated in this work, with a focus on ultrafast excitation energy transfer and charge transfer, which are ubiquitous in Nature and technology. A variety of stationary and time-resolved spectroscopy techniques were used to gain a better understanding of these two processes, for both the intra- and intermolecular case. The studies were not limited to purely organic systems, but also included the interaction of organic molecules with noble metal clusters. In addition, the development and characteristics of a fluorescence up-conversion set-up offering sub-70 fs temporal resolution is also presented.