erictbone,
   Not very much info to go on... a few simple examples would be helpful.
   If your form is like a checkbook...
Trans     Date        Payee       Debits       Credits
21         1/1/08     XYZ Co. 100.00
22         1/2/08    Deposit                      500.00
23        1/10/08   ABC Co.  100.00
                                            -------------------
                                            200.00     500.00      Balance
= 300.00
In the form footer,  = Sum(Debits)    would give you the sum of debits
(200.00)
Same with    =Sum(Credits)     yields 500.00
And Balance would be... = Sum(Credits) - Sum(Debits)

Use the same method in a Report Footer.