© 2015 T.-C. Huang, J.-L. Huang, and K.-T. Cheng. Flexible electronics are emerging as an alternative to conventional Si electronics for smart sensors, disposable RFID tags, and solar cells. By utilizing inexpensive manufacturing methods such as ink-jet printing and roll-to-roll imprinting, flexible electronics can be made on low-cost plastic films just like printing newspapers. However, the key elements of flexible electronics, thin-film transistors (TFTs), have slower operating speeds and are less reliable than their Si electronics counterparts. Furthermore, TFTs are usually mono-type - either p- or n-type - devices. Making air-stable complementary TFT circuits is very challenging or sometimes not feasible to most TFT technologies. Existing design methodologies for Si electronics, therefore, cannot be directly applied to flexible electronics. Other factors such as high supply voltage, large process variation, and lack of trustworthy device modeling also make designing larger-scale and robust TFT circuits a significant challenge. The objective of this article is to provide an in-depth overview of flexible electronics from their applications, manufacturing processes, device characteristics, to circuit and system design solutions. We first introduce the low-cost fabrication methods for flexible electronics, including ink-jet printing, screen printing, and gravure printing. The device characteristics and compact modeling of several major TFT technologies will be illustrated. We will then give an overview of digital and analog circuit design from basic logic gates to a microprocessor, as well as design automation tools and methods, for designing flexible electronics. We also describe a reliability simulation framework that can predict TFT circuits' performance degradation under bias-stress. This framework has been validated using the amorphous-silicon (a-Si) TFT scan driver for TFT-LCD displays. Finally, we will give an overview of flexible thin-film photovoltaics using different materials including amorphous silicon, CdTe, CIGS, and organic solar cells.