Total Harmonic Distortion (THD) is a critical parameter for LED drivers, as it measures the degree to which the input current waveform deviates from a pure sine wave. High THD can lead to poor power quality, overheating, and reduced lifespan of electrical equipment. For LED lighting systems, regulatory standards such as IEC 61000-3-2 and ENERGY STAR impose strict THD limits to ensure grid stability and energy efficiency. Typically, for LED drivers rated above 25 watts, the THD should be less than 20% at nominal load, while some premium designs target below 10% to meet residential and commercial requirements. Lower THD not only reduces harmonic pollution but also improves power factor and reduces neutral conductor currents. Designers must consider circuit topology, input filtering, and control algorithms to achieve compliance. For example, active power factor correction (PFC) circuits can significantly reduce THD by shaping the input current waveform. Additionally, using high-quality magnetics and optimized switching frequencies helps minimize high-order harmonics. Understanding these limits and their impact on system performance is essential for developing reliable, safe, and environmentally friendly LED drivers. By adhering to THD standards, manufacturers can enhance product reliability, avoid penalties from utility companies, and contribute to cleaner power grids worldwide. Ultimately, keeping THD within regulatory limits is a key requirement for modern solid-state lighting.