......Have to be linked?

I suspect yes, or Crystal could get confused.

I have taken over a report from someone else and I see they have multiple tables / command but are not all linked. 

Surely they should be?

If not, how does Crystal know what is the main data to report on?

TIA

DC

If there is a parameter that will select a specific record in an unlinked table, this can work - I've used this technique in several places.  However, if that is not the case you end up with cartesian joins to the unlinked tables which can cause problems on the database as well as too much data to be selected.