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A frequently asked question presented to the journal is this: how many lines of evidence-one, two, or three-are required by regulatory staff when evaluating an MNA remedy? For this issue, we invited six state regulators to respond as guest panel members to provide their perspectives on this topic. In addition, we solicited responses from members of the journal's advisory board with consulting experience, Joseph E. Odencrantz and John A. Simon.Their answers have not been coordinated or edited for consistency and represent their opinions based on experience and expertise. Again, we encourage you to send your thoughts or comments on MNA topics to foster further discussions for future issues of the journal. Questions may be submitted to Mark Ferrey at mark.ferrey@pca.state.mn.us and/or Bob Norris at nrkathleen@quest.net. Question: In 1993, the National Research Council (NRC) Committee on In SituBioremediation developed guidelines for evaluating in situ bioremediation or natural attenuation as a remedy (NRC, 1993), specifying that three discrete lines of evidence should be documented in support of a decision to adopt natural attenuation as a remedy for a site:• A loss of contaminants (reduction in mass/degradation) should be demonstrated.• Laboratory assays or technical literature must show that microorganisms from site samples have the potential to transform the contaminants under the expected site conditions. • One or more pieces of information should demonstrate that this biodegradation potential is actually realized in the field.