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The most commonly used optical detectors for fiber optic sensors are semiconductor photodiodes and avalanche photodiodes (APDs). These types of detectors are generally used to monitor power returned from the sensor. This chapter discusses the statistics of optical detection to provide an overview of optical detection and explains the basic principles of semiconductors, photodiodes, APDs, noise, and spectral detection. The process of optical detection involves the conversion of optical energy in the form of photons into an electrical signal in the form of electrons that can be processed by conventional signal-processing electronics. Wavelength-encoding fiber optic sensors require the detection of the spectral components of the optical signal returned to the signal processing location. The problem of noise in an optical detector relates to the resolution, accuracy, and dynamic range available in a detected signal. The chapter examines the mathematical basis of noise and also discusses the various noise types in turn.