:Four generations of poly(propylene imine) dendrimers have been covalently modified with pyrene moieties and examined by fluorescence spectroscopy. Emission and excitation spectra were obtained so that the generational dependence of the dendrimers, especially the extent of steric crowding, could be correlated to the photophysical properties of pyrene. In particular, the effect of dendrimer concentration and generation on pyrene-pyrene interactions was evaluated. More excimer emission was observed for higher generation dendrimers, while little or no evidence for interdendrimer interactions was observed over more than 2 orders of magnitude in dendrimer concentration. Excitation spectra revealed the presence of preassociated pyrene moieties. Protonation of the tertiary amine units was shown to increase monomer fluorescence significantly, while only slightly increasing the observed excimer fluorescence.