An operational definition for success of in situ bioremediation at field scale includes meeting regulatory goals for ground water quality in a timely fashion at a predictable cost. Current practice for site characterization does not adequately define the amount of contamination subject to bioremediation. As a result, laboratory estimates of the requirements for electron acceptors and mineral nutrients and of the time required for remediation have much uncertainty. In contrast to laboratory studies, the extent of remediation achieved at field scale is influenced by dilution of compounds of regulatory concern in circulated water and by partitioning of the regulated compounds between water and residual nonaqueous-phase oily material. The extent of weathering of residual oily-phase material and the hydrologic environment of the residual have a strong influence on the potential for ground water contamination after active remediation ceases. (Copyright (c) 1993 National Academy of Sciences.)