The chapter analyzes three types of computer systems from the hardware and software point of view, slot machines, respectively. The owners of these devices, by different methods of illegal manipulation, can have total control of the chances of winning and losing, and on the other hand, they can determine at will the amounts to be recorded in accounting. The investigation techniques used in the analysis of electronic gaming machines with winnings show that they present particular problems of cyber security. As a result, solutions have been proposed to solve these problems, such as those concerning the verifications that the legal metrology offices must carry out.