The knowledge of the signals and systems of a technical process and their abstract mathematical description are part of system theory. This textbook provides an introduction to system theory and delivers the foundational elements needed to capture, assess, and design a wide range of applications from a system-theoretical perspective. The main chapter on signals addresses both continuous and discrete signals. After introducing the Fourier transformation, the description of signals and systems in the frequency domain follows. The transition from continuous to discrete signals and the explanation of their spectra also introduces sampling as the transition from continuous to discrete signals. The fast Fourier transformation is derived in detail and in an understandable manner. Other focal points include the distribution and the distribution density function, as well as statistical description.