Exciton migration and annihilation dynamics in J aggregates comprised of a pseudoisocyanine (PIC) dye and poly(vinyl sulfate) (PVS) are studied by time-correlated, single-photon-counting (TCSPC) methods coupled with near-field scanning optical microscopy (TCSPC-NSOM) and also by conventional, far-field TCSPC. Far-field TCSPC studies of the aggregates in aqueous solution demonstrate that the exciton lifetime in the absence of annihilation (at low excitation intensities) is 789 +/- 36 ps with biexponential exciton decay occurring at high excitation intensities due to exciton annihilation. Analysis of the intensity-dependent decay with conventional exciton decay theories demonstrates that the observed decay dynamics are consistent with exciton migration limited to finite molecular domains. In contrast to the solution studies, deposition of a thin PIC/PVS aggregate film onto fused quartz results in a dramatic reduction in the exciton lifetime to similar to 10 ps. This lifetime demonstrates only modest dependence on excitation intensity. Possible mechanisms for the reduction in exciton lifetime for the adsorbed aggregates are discussed. Finally, the dependence of the exciton lifetime on the nanostructure of the aggregates is explored with TCSPC-NSOM. A new TCSPC-NSOM instrument with an instrument response of 30 ps and spatial resolution of similar to 100 nm is presented and used to perform the first direct measurement of exciton lifetimes for a single aggregate. TCSPC-NSOM studies demonstrate that the exciton lifetime is not sensitive to the structural details of the aggregates on the similar to 100 nm length scale. The combination of measurements presented here demonstrate that the exciton diffusion length is limited to short distances (similar to 100 nm) relative to the aggregate size. The homogeneous spectral properties observed for these aggregates are therefore not due to site-averaging resulting from long-range exciton migration but instead are due to the structural homogeneity of the aggregates on the length scales investigated here.