An introduction to quantum-dot cellular automata (QCA) is presented along with experimental implementations. QCA is a transistorless nanoelectronic computation paradigm that addresses the issues of device and power density, which are becoming increasingly important in the electronics industry. Scaling of CMOS is expected to come to an end in the next 10-15 years, with perhaps the most important limiting problem being the power density and the resulting heat generated. QCA offers the possibility of circuitry that dissipates many orders of magnitude less power than CMOS, is scalable to molecular dimensions, and provides the power gain necessary to restore signal levels.