I am pretty new to ECS so perhaps I am not thinking correctly about the problem and how it should be solved in an ECS way. But I really want to tie my Systems to certain Scenes in Unity. This seems logical to me, why would I want my startup Scene Systems running while I am in my Game Scene, etc. Perhaps I should think of Systems as more expressly tied to Data but even still, there is likely data that is only in the Startup Scene and not in the Game Scene and vice versa. Why run systems on data that I know wont be present. Has anyone else had an issue with managing Systems (when and where they should run)? To accomplish this it looks like I really should declare all of my systems [DisableAutoCreation] and then in the Start() method of a monobehavioir attached to the Scene manually insert the systems I want. I am not precisely sure how to do this but roughly I think I just need to get SimulationSystemGroup from World.Active and then just do a SimulationSystemGroup.AddSystemToUpdateList for all systems (or more likely SystemGroups) that I want to add to the scene. As an aside I am not sure I really like the whole instantiate every system from the assembly as the default way of doing things. It means that if any asset creator implements a system, that system is running in my scene whether I declare it to or not. Just by way of adding something from the asset store to the project its now running in all active scenes.I think I would have rather had something more explicitly declared.