The question of how to proceed toward ever more realistic plasma simulation studies using ever increasing computing power is addressed. The answer presented here is the M3D ͑Multilevel 3D͒ project, which has developed a code package with a hierarchy of physics levels that resolve increasingly complete subsets of phase-spaces and are thus increasingly more realistic. The rationale for the multilevel physics models is given. Each physics level is described and examples of its application are given. The existing physics levels are fluid models ͑3D configuration space͒, namely magnetohydrodynamic ͑MHD͒ and two-fluids; and hybrid models, namely gyrokinetic-energetic-particle/MHD ͑5D energetic particle phase-space͒, gyrokinetic-particle-ion/ fluid-electron ͑5D ion phase-space͒, and full-kinetic-particle-ion/fluid-electron level ͑6D ion phase-space͒. Resolving electron phase-space ͑5D or 6D͒ remains a future project.Phase-space-fluid models are not used in favor of ␦ f particle models. A practical and accurate nonlinear fluid closure for noncollisional plasmas seems not likely in the near future.

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