In this chapter the techniques used for describe the performance of devices in a system will be considered. To this aim it is important to consider the system as a combination of elementary devices that follow a well-defined functional structure. After a brief introduction, the reliability evaluation of series, parallel and mixed structures will be shown and discussed. To this aim, the concept of Reliability Block Diagram is also defined as a mandatory tool. The theory is developed using many practical examples. Parallel configuration is further developed in order to discuss the different type of redundancy for reliability growth: active, warm and standby. At the end of this chapter two different types of redundancy approaches are compared: system redundancy and component redundancy. The results so obtained are fundamental during the design phase of a system when reliability aspects have to be taken into account.