Optical pump-probe (transient absorption) spectroscopy was used to study excitation energy transfer (EET) in random molecular systems and in antenna complexes of photosynthetic organisms. Energy transfer was first studied for Rhodamine 640 adsorbed on ZnO and fused silica surfaces at submonolayer coverage. Ground state recovery on fused silica was coverage-dependent due to excitation trapping by dye aggregates; on ZnO recovery was fast and independent of coverage. This can be explained by energy transfer into semiconductor surface states. Also, EET experiments were performed on solutions of Rhodamine 640 in glycerol using both absorption depolarization and the well-known technique of fluorescence depolarization using time-correlated single photon counting. Data from both techniques match within experimental error theoretical predictions, showing the validity of pump-probe techniques in studying EET;The absorption depolarization of the Q[subscript] x electronic transition in BChl a-protein from P. aestuarii was monitored, and the anisotropy was found to decay to a constant, nonzero value due to nonrandom chromophore orientations. The timescale of depolarization was found to be consistent with transfer between delocalized exciton states. Results of experiments on the core antenna complex of photosystem I were remarkably similar, showing that local ordering exists in PS I. This also suggests that EET occurs between groups of chromophores in PS I similar to those in the BChl a-protein. ftn[superscript]1DOE Report IS-T-1383. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.