This chapter introduces sensor as a device (sort of black box approach) and describes various characteristics that define such a sensor and most importantly how to interpret its performance in actual use employing these characteristics. Depending on the actual sensor and its intended application, some of these characteristics become relevant and some are not. For example, the dynamic characteristics of a sensor (load cell) used in a stand alone weighing machine is not at all relevant and just the static characteristics will do. On the other hand if the same sensor is used to weigh objects on a conveyor belt, then the dynamic characteristics of the sensor along with the static characteristics have to be considered. Hence the reader must exercise caution and not blindly insist that a sensor must possess all the characteristics described herein. © Springer International Publishing AG 2017.