Excluding some records

I have 2 database tables with a common field ([Reference Number]).
Table 1 has all of the reference numbers (in most cases multiple entries), whereas Table 2 only contains some of these and has only one entry in all cases.
I would like a query to show only those Reference Numbers from Table 1 that do not appear in Table 2.

Have just found the query I was looking for in the design wizard, didn't think of looking there first.