Question : Sum based on condition

I am using SQL Reporting Services 2005 and have a column on my report that I need to sum based on a condition. Example: If [DM State] = DM then Sum [Invoice].
My data set contains DM State, Invoice, MonthVolume. I have a group on MonthVolume, and have a secondary header containing my totals, which is where I need to place this conditional sum.
Thanks!!

Answer : Sum based on condition

what are the field names...

=IIF(Fields!State.Value="DM", SUM(Fields!Invoice.Value), 0)