Although an add-in is a hidden, read-only workbook, you can programmatically modify it. For example, you can add a worksheet on which you can perform calculations and store values. However, changes to an add-in are temporary — when you close the add-in, or when the add-in is unloaded from memory when you quit Microsoft Excel, the changes are lost. If you require more persistent storage, you can use your add-in to create a new workbook, transfer data into it, and then save it.