In this contribution, basic aspects of modern sensors and especially biosensors are addressed. The most important terms will be defined and a number of transducing principles will be illustrated. For biosensors, the role of the biochemical recognition element and the immobilization of biomolecules on the sensor surface will be emphasized. Glucose sensors will serve as an example to illustrate the development of the past decades. In this context also the roles of nanotechnology and nanomaterials for the development of sensors and biosensors will be highlighted. Thereafter, some examples of label-free optical and electrochemical biosensors are presented. Finally, the possibilities of integrated and miniaturized biosensors are discussed, a development which will eventually result in so-called labs-on-a-chip.