We describe weaknesses in most master-keyed lock systems, such as those used by offices, schools, and businesses as well as by some residential facilities (particularly apartment complexes, dormitories, and condominiums). These weaknesses allow anyone with access to the key to a single lock to create easily the "master" key that opens every lock in the entire system. Creating such a key requires no special skill, leaves behind no evidence, and does not require engaging in recognizably suspicious behavior. The only materials required are a metal file and a small number of blank keys, which are often easy to obtain.