This is a first post from a series devoted to building xll add-ins for Excel. Add-ins are used to extend Excel functionality. It is possible to extend Excel using VBA in the same spreadsheet. However this way VBA code is tightly coupled with the spreadsheet, which makes it difficult to keep track of code versions or update code for all copies of a spreadsheet. Users often make copies of spreadsheets because they want to modify something but want to keep original around.