After not wanting to, it looks like I do need to create a
macro. How would I do the following to make sure that it
captures a range of missing numbers (1006-1008 below)? I
need to find all gaps in the Invoice # list and flag the
numbers to a new worksheet(#2), preferably all listed in
Column A without gaps.

Worksheet #1 Worksheet #2
Invoice# Customer Invoice#
1001 Acme 1003
1002 Davis 1006
1004 Shopstar 1007
1005 Walker 1008
1009 Trains

Thanks for any help.