In this chapter, we discuss different types of photodetectors, plastic scintillators and properties of time measurements. The most commonly used instruments for time measurements consist of fast scintillators read out by photomultiplier tubes (PMT). Though silicon-based devices, such as the SiPM, are used more often today for time measurements, photomultiplier tubes are still important photodetectors since they record photons with high efficiency and excellent time resolution. They are used in Cherenkov detectors, trigger counters and calorimeters. Plastic scintillators are used in sampling calorimeters, trigger counters and time-of-flight detectors.