When using a Settings.settings file in .NET, where is the config actually stored?

It depends on whether the setting you have chosen is at "User" scope or "Application" scope.

User scope

User scope settings are stored in

C:\Documents and Settings\ username \Local Settings\Application Data\ ApplicationName

You can read/write them at runtime.

For Vista and Windows 7, folder is

C:\Users\ username \AppData\Local\ ApplicationName

or

C:\Users\ username \AppData\Roaming\ ApplicationName

Application scope

Application scope settings are saved in AppName.exe.config and they are readonly at runtime.

Two files: 1) An app.config or web.config file. The settings her can be customized after build with a text editer. 2) The settings.designer.cs file. This file has autogenerated code to load the setting from the config file, but a default value is also present in case the config file does not have the particular setting.