label expression: 'if not field 1, then field 2', how is this achieved

Hi,
I am looking to create a label expression that will result in the following: display 'field 1', however if 'field 1' is blank, display 'field 2'
I am sure this can be written a number of ways to get the same result (eg other way round: display 'field 2' unless 'field 1' is not blank)
Would anyone be able to show how to do this?

thank you very much- any help would be greatly appreciated.

JB