Sensors, transducers, and actuators are integral parts of any kind of system for monitoring, feedback, and control purposes. The basic components of any sensing device consist of an active region to detect the target analyte and a signal transducer to convert the detected quantity into a comprehendible signal. An external or internally integrated signal processing unit then translates and converts that signal to be used directly by an operator or a machine. Based on the readings provided by the sensors or through the user input, any kind of physical motion is then controlled by the actuators. There are a huge number of sensors, transducers, and actuator types based on their structures, materials, working principles, and applications. The common thing they all possess, though, is that they have similar target performance parameters. Any category of sensors or actuators will ideally have low cost, easy fabrication, high stability and robustness, high sensitivity, wide range, high precision and accuracy, and low hysteresis. Based on the above-mentioned performance parameters and the application-specific requirements, different types of sensors, transducers, and actuators are selected for different kinds of systems. The defining parameters and the possible relationships between the target system requirements and the selection process for the appropriate devices will be discussed in detail in the different sections later in this chapter.