>>>This stored proc takes 2 optional params.
>>>
>>>

>>>
>>>CREATE PROCEDURE bm_GetClients
>>>	@CompanyKey		INT = NULL,
>>>	@ClientKey		INT = NULL
>>>
>>>AS
>>>BEGIN
>>>
>>>	SELECT *
>>>		FROM bm_Clients
>>>		WHERE CompanyKey = ISNULL(@CompanyKey, CompanyKey) OR
>>>			  ClientKey = ISNULL(@ClientKey, ClientKey)
>>>
>>>END
>>>

>>>
>>>I want to pass either, both, or none. I'm running it like this:
>>>
>>>

>>>DECLARE @CompanyKey	INT
>>>DECLARE @ClientKey	INT
>>>SET @CompanyKey = 2
>>>SET @ClientKey = 5
>>>
>>>EXEC bm_GetClients
>>>EXEC bm_GetClients @ClientKey
>>>EXEC bm_GetClients @CompanyKey, @ClientKey 
>>>EXEC bm_GetClients @CompanyKey
>>>

>>>
>>>In all cases I'm getting back all records
>>
>>Change:
>>

>>WHERE CompanyKey = ISNULL(@CompanyKey, CompanyKey) OR
>>      ClientKey  = ISNULL(@ClientKey, ClientKey)
>>

>>
>>to
>>

>>WHERE CompanyKey = ISNULL(@CompanyKey, CompanyKey) AND
>>      ClientKey  = ISNULL(@ClientKey, ClientKey)
>>
>>

>
>If I change the OR to AND, then the second run returns no records.


Nope,
It should because if the first parameter is NULL this:
CompanyKey = ISNULL(@CompanyKey, CompanyKey) will be always TRUE

BTW You should use Named parameters if you want to pass only second one or use NULL as first parameter:

DECLARE @CompanyKey	INT
DECLARE @ClientKey	INT
SET @CompanyKey = 2
SET @ClientKey = 5

EXEC bm_GetClients @ClientKey = @ClientKey
-- or
EXEC bm_GetClients NULL, @ClientKey