Ultra-Low-Power Subthreshold Circuit Design is a circuit methodology operating transistors below normal threshold voltages enabling exponentially reduced power consumption at the cost of reduced speed and increased process sensitivity — Subthreshold operation leverages the exponential current-voltage relationship of MOSFET devices, enabling orders-of-magnitude power reduction for energy-constrained applications. Transistor Physics exploits exponential subthreshold current dependence on gate voltage, trading speed for power since lower voltages reduce current and frequency. Power Benefits deliver power consumption primarily from subthreshold leakage current avoiding dynamic switching power, enabling microwatt and nanowatt operation. Speed Trade-offs accept reduced circuit speed operating at kilohertz frequencies compared to gigahertz conventional operation, suitable for energy-constrained sensors and biomedical devices. Voltage Scaling reduces supply voltages to 0.3-0.5V from nominal 1.8-3.3V, enabling near-threshold operation maximizing energy efficiency. Device Sizing requires larger transistors compensating reduced transconductance, increasing area and capacitive loading. Variability Management addresses increased process variations in subthreshold region causing frequency and threshold voltage spreads, requiring robust design and adaptive techniques. Circuit Topologies employ differential pairs, cascode structures, and current mirrors adapted for subthreshold operation. Applications include biomedical sensors harvesting microwatts, wireless sensor networks requiring months of battery operation, and implantable devices demanding tiny power budgets. Ultra-Low-Power Subthreshold Circuit Design enables perpetually-operating autonomous systems.