WHERE Statement On Two Tables?

How do I do a WHERE statement on two tables?

CREATE TABLE [Audit]
(
[AuditID] [int],
[AuditDate] [datetime]
)

CREATE TABLE [ToDoItem]
(
[ToDoItemID] [int],
[AuditID] [int],
[DueDate] [datetime],
[CompletedDate] [datetime]
)

What I need to do is select all Audits that
still have ToDoItems not complete
that the DueDate has passed
or where the audit was done over six months ago

What I tried is

SELECT
[AuditID]
FROM
Audit a
WHERE
(a.AuditID in (SELECT AuditID
FROM ToDoItem t
WHERE
t.CompletedDate is null
AND (t.DueDate < getDate()
or ( (DateAdd(Month, 6, < a.AuditDate)) < getDate()) )
))


----
Dave