(I previously posted this as a reply to an earlier post but I'm hoping that
someone new might see this.)

The array formula was extremely useful and I am trying to edit it for the
following:

Gaps sheet is where account and meter info is kept, along with periods
(months) that have already been reported.

Main sheet is where new data is kept. I want to delete the rows where those
periods (months) have already been reported.

Gaps!A3:A10 = account #
Gaps!C3:C10 = meter #
Gaps!E3:E10 = reported periods
main sheet, cell A2 = account #
main sheet, cell F2 = meter #
main sheet, cell P2 = period that this billing data is for

=INDEX(Gaps!$E$3:$P$10,MATCH(1,(Gaps!$A$3:$A$10=A2 )*(Gaps!$D$3:$D$10=F2),0))

I would like to mark rows that should be deleted with "Yes" (must be
deleted) or "No" (stays).

Should I be using a different function or are my operators incorrect? What
am I missing? Please let me know if I need to give further clarification.
HELP!!!

Thank you all for any suggestions.