missing master records

Anybody out there knows how to select records from one table (table 1) which has a common key with another table (Table 2), and where the record exists in table 1 but not in table 2 ? Eg. I've got a master invoice table "invmst", and a invoice line items table "invlin". They have a common key "invoice". I want to find records in the "invlin" table which has no corresponding "invoice" number in the "invmst" table. I've seen the solution somewhere, using a nested select, but I have not been able to dig it up since.