Mark,If any tool, (tool, model or script) inside the main model has the checkbox "always run in foreground", than the main model inherits from this. Thats why you see it checked and greyed out.You'll have to set the checkbox appropriately for your sub-models and scripts. For system tools you cannot set this. Either a tool is capable of running the background or not. Hopefully all the system tools you're using are "background capable"