Disturbances in power supply to buildings can significantly affect the reliability, safety, and integrity of not only the electrical distribution system components but also of the connected loads. This chapter outlines common power quality disturbances and means to mitigate them in order to minimize significant damages to the building electrical systems and maintain reliable performance of electrical equipment. Voltage sags, voltage swells, interruptions, transients, voltage unbalances, harmonic distortions, and electrical noises are some of the power quality disturbances common in buildings and are presented in this chapter. In particular, a detailed description of the harmonic distortions is provided, with specific metrics to assess their magnitudes and mitigation methods to reduce their effects.