To link to an Excel file, you'd use a LINK field. Simply copying & pasting from Excel, using 'paste special' with the 'paste link' option will give you both the link and a choice of paste formats.

The key to the method's success in this case is to not expect Word to find a different set of cells for each period; if necessary, create an output sheet with whatever linking formulae are required to retrieve the data of interest into a set of cells that can be used for any month.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]