Development of the comprehensive codes used to study the boundary region of the DIII-D tokamak has been done in parallel with improvement of the diagnostics of this important region of the plasma. These codes have been used to interpret the diagnostic data and to assist in the design of improved divertor configurations. The development of codes used for analysis on DIII-D is described briefly. Model validation by comparing with the extensive DIII-D boundary region diagnostic data is also discussed.