selecting all records where a column is null

Hi, I am trying to select every record from a table where a specific
column does not have a value in it yet.  Is there a way to express that
in a SQL statement?  Here are the few I tried:
 
select * from table where field;
select * from table where field = '';
select * from table where field = NULL;
 
and some other stuff the is way more broken.  I have flipped through the
docs a bit but I haven't found anything useful yet.  If someone could
point me to the right place in the docs or give me an example of a
select that will do this I would be very grateful.
            --Erik