This paper presents the sliding mode control (SMC) of unified power quality conditioners (UPQC) intended to compensate power quality issues in three-phase four-wires systems. The SMC UPQC can be applied in electrical grids or isolated grids to mitigate power quality problems at the consumer facilities and also to minimize issues for the grid supplier. The UPQC is configured as a shunt-series filter. The shunt Active Power Filter (APF) uses a three-phase rectifier with SMC to enforce sinusoidal mains currents. The series APF uses three single-phase H-bridge inverters and SMC to improve the voltage quality at the point of common coupling. SMC design, analysis and simulation results are presented and discussed.