Energy Harvesting Circuit Design is a specialized circuit methodology capturing ambient or residual energy from environmental sources and converting it to usable power for autonomous devices — Energy harvesting enables perpetual operation of wireless sensors, medical implants, and remote IoT devices through ambient energy sources eliminating battery replacement. Energy Sources include solar radiation harvesting through photovoltaic cells, vibration through piezoelectric or electromagnetic transducers, thermal gradients through thermoelectric generators, and RF signals through rectenna antennas. Photovoltaic Harvesting implements maximum power point tracking adjusting load impedance for optimal power extraction, buffering variable solar output through charge storage, and managing voltage variations across lighting conditions. Vibration Energy converts mechanical motion through piezoelectric devices generating voltage or electromagnetic induction generating current, requiring impedance matching and frequency tuning for optimal power. Thermal Energy exploits temperature gradients across Seebeck junctions, optimizing thermal coupling and impedance for maximum power transfer. RF Energy rectifies ambient electromagnetic signals through efficient rectifier designs, implements impedance matching networks, and manages receiver sensitivity versus power extraction trade-offs. Power Conditioning includes voltage regulation maintaining stable supply from variable harvested sources, efficient DC-DC conversion minimizing losses, and energy storage management. Storage Elements employ supercapacitors providing rapid charge/discharge cycling, rechargeable batteries managing limited cycles, or hybrid approaches optimizing cycle life. Energy Harvesting Circuit Design enables truly autonomous IoT systems.