This introduction to the practice of calculating the reliability and availability of technical systems places particular emphasis on the technical and economic benefits of the results. A device or system is considered reliable if it performs its defined function over a certain period of time. Availability is defined as the probability that a system is operational at any given moment, or, from another perspective, that it is available, for example, 99.999% of the time. Both metrics are determined using statistical methods. The methods are presented in detail and, where appropriate and necessary, are supported by the relevant mathematics and examples. The necessary data is defined, and the results obtained are interpreted.