Abstract: The promise of nanotechnology lies in the emergence of novel electronic and photonic phenomena with the potential for new device technologies. For example, optical transitions at the nanoscale frequently result from strongly absorbing excitons, whose electronic structure and dynamics are shape, size, and morphology dependent. However, since excitons are bound states, device concepts that exploit the unique photophysics of excitons, for example, organic photovoltaics or multiple exciton generation solar cells, depend critically on the ability to direct specific favorable conversion processes and suppress unfavorable ones. Here I will discuss how ultrafast optical spectroscopy is an important tool to understand the success (and failure) of nanoscale device concepts. I will demonstrate how the unique optical signature of excitons allow for dynamical tracking of energy conversion processes on ultrafast time scales. These optical signatures can be used to understand the harvesting of excitons in nanoscale lasers, photocatalytic water splitting devices, and photovoltaics. I will show how the first few picoseconds after light absorption are crucial to understanding the overall performance of a nanomaterial-based energy device.