Flexible electronics are emerging as an alternative to conventional Si electronics for large-area low-cost applications such as e-paper, 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 less reliability than their Si electronics counterparts. Furthermore, depending on the material property, TFTs are usually mono-type - either p- or n-type - devices. Making air-stable complementary TFT circuits is very challenging and not applicable to most TFT technologies. Existing design methodologies for Si electronics, therefore, cannot be directly applied to flexible electronics. Other inhibiting factors such as high supply voltage, large process variation, and lack of trustworthy device modelling also make designing larger-scale and robust TFT circuits a significant challenge. This book provides an in-depth overview of flexible electronics from their applications, manufacturing processes, device characteristics, to circuit and system design solutions. It first introduces the low-cost fabrication technology for flexible electronics, including ink-jet printing, screen printing, and gravure printing. The device characteristics and compact modelling of four major types of TFT technologies, including hydrogenated amorphous silicon (a-Si:H) TFT, polymer organic TFT, self-assembly monolayer (SAM) organic TFT, and metal oxide TFT, are illustrated. The book then gives an overview of digital and analog circuit design for flexible electronics from basic logic gates to a microprocessor. In order to accurately predict the time-dependent degradation of TFT circuits, it describes a reliability simulation framework that can predict the 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, the book provides an overview of flexible thin-film photovoltaics using different materials including amorphous silicon, CdTe, CIGS, and organic solar cells.