Signals can be found in very different forms as carriers of information. Large amounts of data must be processed in such a way that they can be efficiently transmitted over a given transmission channel or stored on a storage medium. To do this, it is necessary to describe signals in the time and frequency domain. Signals can have deterministic or random (or stochastic) characteristics; moreover, they can be analog or digital. The processing and linking of signals for information transmission and storage is done with the help of suitable systems. Problems of information and signal transmission can therefore usually be traced back to questions that concern the evaluation and transformation of signals by systems. This chapter provides an overview of the possibilities for describing signals and systems and presents the basics necessary for understanding telecommunications contexts, which are dealt with in the following chapters, using the time and frequency domain representation including the necessary transformations.