Laser phase noise is a frequency-domain view of the noise spectrum around the laser signal. It is related to fluctuations of the optical phase of the laser's output. Paschotta has a particularly strong expertise in this area. In some important application areas such as laser material processing, noise properties may be of li-mited interest, although even in that area there can be. ifold influences on ap plications. Here we discuss where such noise can come from, how it is quantified and how its influences can be mini-mized. For example, interferometric measurements are sensitive to optical phase fluctuations, while intensity fluctuations can limit.