Flexible electronics has emerged as a transformative technology, enabling devices that combine high performance and durability with properties such as bendability, stretchability, and the ability to conform to various shapes. The development of these devices requires the use of materials and substrates with high flexibility and excellent physical and chemical characteristics. This chapter provides a review of the important properties required for flexible substrates for electronic devices and gives an overview of the most used substrate materials and their characteristics. A background and review of research and advancements in key flexible electronic devices, including solar cells, displays, transistors, and sensors, with a focus on their applications in portable and wearable electronics, are presented. Particular attention is given to polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) films, which have become the substrates of choice compared to other plastic alternatives. Additionally, the chapter briefly discusses unconventional flexible substrate materials that emphasize biodegradability, enhanced flexibility, and compatibility. These materials can open the pathway for expanding the functionality and sustainability of flexible electronics.