This introduction presents an overview of the key concepts discussed in this book. The book introduces the fundamental physics of electrical components such as flexible and stretchable transistors, memories, and sensors to understand the present status of the flexible and stretchable electronics including techniques to fabricate the devices as emerging technologies. Flexible devices such as transistors and transducers have been developed since 1990 by forming the organic thin film or transferring inorganic thin film on plastic substrates. The book describes the medical and healthcare applications using the flexible and stretchable device components. It discusses the skin-mounted healthcare devices, implantable medical devices, and neuroscience devices. In addition to the device applications, the book also discusses the nanosheet materials for the practical medical and wearable sheets. Recently, fully printed, multifunctional, low-cost, wearable flexible healthcare devices have been reported by integrating with multiple sensors for healthcare detections and human activity.