A performance assessment methodology has been developed for use by the US Nuclear Regulatory Commission (NRC) in evaluating license applications for low-level waste disposal facilities. This paper provides a summary of the modeling approaches selected for the methodology, and includes discussions of the philosophy and structure of the methodology. The performance assessment methodology is designed to provide the NRC with a tool for performing confirmatory analyses in support of license reviews related to postclosure performance. The methodology allows analyses of dose to individuals from off-site releases under normal conditions as well as on-site doses to inadvertent intruders. Computer codes have been chosen for source-term analysis, for vadose-zone flow and transport, for saturated-zone flow and transport, for surface-water transport, for transport via the air pathway, and for food chain and dosimetry analyses. More than one computer code is recommended for each module in the methodology, which increases its flexibility, but also increases the amount of user effort required to provide interfaces between the modules. 25 refs., 2 figs.