With the advancing technology, loads in the system have been varied. These loads are always asked to be linear. However, with the effect of progressive semiconductor technology, an increase in nonlinear loads has been observed in the system. Nonlinear loads are loads whose current and voltage characteristics are nonlinear. Nonlinear loads cause harmonic currents and voltages in the system. Major loads causing harmonics are power electronic components, transformers, uninterruptible power supplies (UPS), converters and high-power induction motors. Harmonics cause additional energy losses in the system, heating and damage to the insulation. For these reasons, it is important to reduce or eliminate harmonics after formation or to block them before the formation. The most important method for eliminating harmonics is the filtering of harmonics with filters. In this study, firstly, it is given basic information about harmonics, their effects on energy system, filtering methods. Then, a sample system model is performed in MATLAB program and the effect of passive filters is examined.