Abstract: An apparatus and method for detecting and preventing the unauthorized use of computer programs. The program to be protected is arranged to store first key information and a key means which is physically separate from the program is provided to the customer with each program which is sold. The key means stores second key information, and is adapted for association with a computer in which the program is to be processed. Either the first program routine, or one which is performed before a significant part of the program, is arranged to access the first and second key information and to perform a comparison operation. If the first and second information bear a predetermined relationship to each other, then it is known that the appropriate key means has been deployed by the customer on the computer, and that the use of the program is authorized.