"Not" operator not functioning in t-sql

[Slade]

Ok guys, check this out

select app_given, app_surname, contract.contractID
from contract, progression
where pay_nop like 'PAY'
and
contract.status like 'Active'
and
app_type like 'Apprenticeship'
and
Contract.ContractID = progression.contractid
group by contract.contractid, app_given, app_surname

Ok, this works FINE. But, when I try to add Not..:

Not Contract.ContractID = progression.contractid

It just brings up all of the contract ID's, including the one's in the progression table. What I want it to do is bring up the contract Ids that ARENT in the progression table.

Thanks in advance for your help,

Slade