In this book, a comprehensive illustration of power system harmonic concept is first introduced for the discussion of total harmonic distortion (THD) and power factor (PF) in power systems. Following a brief review of passive filters and well-known active filters, the fundamental control principle and structure of the proposed adaptive filtering system are then presented. Secondly, an enhanced Fourier coefficient identification (E-FCI) algorithm for simultaneous multiple harmonic identification is developed. Based on this E-FCI, a parametric adaptive filter (voltage-type) that achieves efficient harmonic filtering is developed. Other contributions of this research include the design and implementation of non-parametric adaptive filters based on artificial intelligence topologies. The non-parametric adaptive filters, consisting of a variable voltage-type source controlled by neural network controllers as optimization tools, can generate the correct compensation current to compensate for the unstable line current harmonics under various speed operations of a DC variable- speed drive.