The description of the development of data processing at Manco is in three parts. The first relates to the 1960's, when Manco introduced a batch computer system. The second describes the development of that system and the introduction of online interactive working in the 1970's. The third part describes the situation in the 1980's. The case uses hypothetical examples to illustrate security problems in computer-based systems. The final section outlines an approach to computer security. This includes restricting physical access to the computer and terminals, controlling access to programs, online editors, systems utilities and data, file balancing controls, data validation, and error repossessing controls. Other security measures are the separation of employee responsibilities, procedures to enforce the correct operation of the system, effective system design, and continuous monitoring of the system log and system operation. 31 footnotes.