Heterogeneous electron transfer involves the coupling of a dense manifold of highly delocalized electronic levels of the solid state to a discrete molecular state as well as an abrupt change in phase in the reaction coordinate. These features make this problem unique relative to homogeneous solution phase or gas phase reaction mechanisms which involve coupling between discrete states within a uniform medium. Recent advances in time domain optical methods are discussed in the context of studying interfacial charge transfer processes at single crystal semiconductor surfaces as a means to probe the primary processes governing heterogeneous electron transfer. Two distinct boundary conditions are discussed: charge injection into a semiconductor from an adsorbate and charge emission from a semiconductor to an acceptor. The reaction dynamics are investigated using a combination of nonlinear spectroscopies with an emphasis on mapping the electron transport and transfer and investigating the role of nuclear vs electronic relaxation mechanisms in the barrier crossing dynamics. A fundamental understanding at this level seeks to determine the criteria for fully optimizing charge separation at surfaces.