Regulatory acceptance of monitored natural attenuation (MNA) requires demonstration that natural processes, such as sorption and biodegradation, attenuate specific contaminants of concern on a time scale that is comparable to other remediation options while concurrently preventing contaminant migration to site-specific points of contact. Two of the tools used to demonstrate the efficacy of MNA, microcosm experiments and numerical fate and transport modeling, were examined in this study. In the first phase of this work, laboratory microcosm studies were initiated as part of an overall MNA site assessment to determine whether a native microbial consortia collected with a soil sample from a petroleum-hydrocarbon contaminated site was capable of biodegrading specific polynuclear aromatic hydrocarbon (PAH) compounds. Results indicated that selected PAH compounds were biodegraded under simulated natural conditions using oxygen and sulfate as electron acceptors. In the second phase of this study, a numerical experiment was conducted using the three-dimensional, multiple substrate, multiple electron acceptor fate and transport model SEAM3D (Waddill and Widdowson, 1997) to evaluate the impact of including iron(III)-reducing conditions during numerical simulations of natural attenuation. Results for this phase of the study indicated that the mass of hydrocarbon simulated as biodegraded by the iron(III)-reducing population was significantly larger than hydrocarbon biodegradation under aerobic conditions. The final component of research used the SEAM3D model to interpret field observations recorded during a natural attenuation experiment where the fate and transport of selected hydrocarbon contaminants (BTEX and naphthalene) was tracked through an extremely heterogeneous, but well-instrumented test aquifer. Results from the calibrated model for the NATS experiment indicated that the majority of the contaminant remained in the non-aqueous phase during the first year of the experiment, and that aerobic biodegradation was the dominant natural attenuation process. Model results were particularly sensitive to the rate of contaminant release and the starting mass of electron acceptor.