Scanning tunneling microscopy has been employed to characterize the microporous and mesoporous structure of different carbon materials. First, model micropores artificially created on a highly oriented pyrolytic graphite substrate by plasma treatment were studied at the atomic scale: the observed increase in electronic density near the Fermi level around the defect implies an increased adsorptivity of the model micropore region. Second, activated carbon fibers were studied. A spongy mesoporous texture along with slit-shaped microporosity (similar to1 nm) was observed, accounting for the high adsorption properties of this material. For comparison, nonporous thermally treated carbon black and nonactivated carbon fibers with a ultramicroporous texture were also investigated. In the former case, as expected, no sign of extensive microporosity or mesoporosity such as that of the activated carbon fibers was encountered, in agreement with its poor sorptive capability. In the latter case, the interpretation of the results was rather troublesome, since the minute pore size rendered a reasonably accurate STM imaging difficult.