I have a workbook with 12 worksheets, one for each month of the year. I also
want to add a "year to date" worksheet. Is there a simple way to take the
sum and/or average of a specific cell, over the 12 month period, and carry it
over to the same cell on the YTD worksheet?

Example: I have a "total pay" in cell A1, on each of the 12 worksheets. I
want the sum of all of the A1 cells to compute in cell A1 on the YTD
worksheet.