The IIS application isolation mode that you choose affects performance, reliability, security, and feature availability. Worker process isolation mode is the recommended mode of operation for IIS 6.0 because it provides a more robust platform for applications and the ability to isolate applications in separate application pools without incurring a performance penalty. Also, in the default configuration, worker process isolation mode provides a higher level of security than does IIS 5.0 isolation mode because its default host process identity is Network Service.Some applications might have compatibility problems when they run in worker process isolation mode. If compatibility problems occur, you might need to use IIS 5.0 isolation mode. For applications that run in IIS 5.0 isolation mode, Local System is the only available process identity when you run an application in-process in Inetinfo.exe. For more information about IIS security, see Managing a Secure IIS 6.0 Solution