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Abstract: In the study of the laser subject to an electrical feedback (see Section 4.1), we assumed that the response time of the feedback was instantaneous. The time to sense information and react to it was neglected because it was much smaller than any time scale of the CO 2 laser. This is, however, not the case for semiconductor lasers (SLs) exhibiting a very short photon lifetime inside the cavity (∼ 10 −12 s) and optical feedback response times 10 3 times larger. In this chapter, we consider a variety of systems in which the dynamics are greatly affected by the response time of the feedback. We first concentrate on the so-called low frequency fluctuations or LFF observed with SLs, because they have been the topic of many investigations in the last 30 years. We first describe the LFF from an experimental point of view and then interpret the phenomenon in terms of numerical bifurcation diagrams. In the second part of this chapter, we show how optical feedback may also be used to improve the sensitivity of imaging systems. The last section is dedicated to optoelectronic feedback systems for which pulsating instabilities appear as a possible source of high frequency (microwave) electrical signals. History Optical feedback (OFB) cannot be fully avoided in experiments. Any optical element placed in front of a laser, such as a detector or even an antire flection coated lens, back-scatters part of the laser beam.