Commercial interest in mechanically flexible plastic electronics is the key motivator behind efforts to fabricate transistors, light-emitting diodes, and lasers from organic thin films. Continued development depends on increasing comprehension of factors affecting charge carrier mobility. In particular, the importance of film microstructure on transport in organic films has been recognized, but is currently not well understood. In this talk, I will describe experiments designed to address microstructural effects on conductivity in polycrystalline organic films. Our approach is to probe transport in individual grains, or even small collections of grains, which we characterize by atomic force microscopy (AFM). Experiments have focussed on crystalline grains of the molecular semiconductor sexithiophene (6T). Isolated grains of 6T are grown by vacuum sublimation onto SiO2/Si substrates. The crystals range from 1-6 molecular layers (2-14 nm) in thickness with diameters on the order of a micron. In one approach, these thin crystals are contacted with source and drain electrodes fabricated by electron-beam lithography; heavily doped Si underneath the SiO2 serves as a gate electrode. The resulting transistor structures are used to probe field effect conductance and carrier mobility as a function of temperature (5-300K) and the number of discrete molecular layers in the crystals. The second experiment uses a conducting AFM probe as a positionable electrical contact to grains contacted by a fixed electrode at the other end. This configuration allows variation of the tip-electrode separation, yielding the single grain resistivity and an estimation of the organic-metal contact resistance. Resistances associated with defects, e.g., a single grain boundary between adjacent crystals, may also be measured. In both types of experiments, the conjunction of AFM imaging with transport measurements is critical to correlating transport properties with specific microstructures.