The effect of intermolecular forces on energy transfer processes, and subtle changes in molecular energy levels can profoundly influence molecular photochemical and photobiological properties. Molecular aggregates isolated under supersonic jet conditions offer opportunities to examine molcular interactions in a way not possible in condensed media, because small, organized arrangements of molecules can be prepared at very low internal temperatures. Many contemporary spectroscopic experiments seek to understand the detailed structures of small aggregates and the potential energy barriers that restrict internal motion. Internal relaxation processes involving conformational rearrangement and microscopic photochemistry in clusters have successfully been time resolved in several cases. At higher energies, vibrational predissoeiation studies convey useful information about the strengths of intermolecular forces. Experiments of this type are constantly being compared with, and offer new opportunities to refine, procedures for structure calculations. One of the most important currently developing areas in cluster spectroscopy involves direct structure determinations. This has involved complementary experiments in both the areas of high-resolution spectroscopy and picosecond time-domain polarization spectroscopy. This combination of structural and dynamical information offers exciting new opportunities to examine photoprocesses on a microscopic scale.