Modern detectors of electromagnetic radiation in the infrared, VIS, UV spectral regions are designed and fabricated based on suitable junctions or electro-optical devices. The performance of these systems needs to be assessed in terms of accuracy and precision. This is made through figures of merit such as Noise Power Spectral Density, Noise Equivalent Power. Detectors can be classified as photovoltaic or photoconductive devices, which allows to better classify the various noise sources: shot noise, 1/f noise, Johnson noise, generation-recombination noise.