Flexible thermoelectric devices with the advantages of deformability, small size, portability, stability and reliability are widely studied, which can realize the combination of complex heat sources and maintain good thermoelectric conversion under dynamic deformation. Compared with conventional block thermoelectric devices composing of block thermoelectric materials and rigid components, flexible thermoelectric devices are more flexible in adapting to the geometry of the heat source and can collect more energy. Therefore, the development of flexible thermoelectric devices as a research direction with a great application potential in the field of thermoelectric technology has attracted much attention. Flexible thermoelectric devices can effectively adapt to the complex surface shape of the heat source in the real scene, as well as the special conditions caused by dynamic deformation, and meet the unique requirements of flexible electronic devices. As a result, flexible thermoelectric devices have a great potential for some applications, such as versatility, power for small portable devices and sensor applications.In this review, the performance evaluation indexes of flexible thermoelectric devices are introduced, and the key performance, interface factors and the important influence of flexible stability indexes on flexible thermoelectric devices are summarized. This review represents the existing relevant research and summarizes the progress in three areas, i.e., flexible substrate devices, low-dimensional flexible devices, and fiber thermoelectric devices. Flexible substrate device is a typical form with the superior output performance. In contrast to the application limitations imposed by the immutability of bulk thermoelectric materials, the flexible substrate device is initially flexible through a flexible design that combines flexible components with rigid thermoelectric legs. The flexibility and wearability of substrate devices are significantly enhanced via optimizing the adaptability of flexible components and employing systematic designs. Also, some studies focus on special application scenarios, and deeply explore the flexible endowing methods for such devices to achieve optimization and innovation of preparation methods. The advent of low-dimensional flexible devices effectively overcomes the inherent limitations of flexible substrates in improving flexibility, which is reflected in improving the applicability of flexible devices in complex, small and dynamic environments. The different application for different heat flow directions are clarified through the in-depth investigation of the design of external and internal heat flow transfer. In the case of thin films for out-of-plane heat flow transfer, although it is difficult to maintain the temperature difference in the film thickness direction, the films are thin and easy to process, thus having an advantage in small and micro-integrated applications. In the case of thin films for in-plane heat flow transfer, the direction of heat flow is designed to propagate in the in-plane direction, and the temperature difference can be maintained in a large lateral range for efficient conversion of heat energy. In the domain of fiber thermoelectric devices, there exist applications that depend directly on structural flexibility. These applications involve the coating or integration of thermoelectric materials directly onto a fiber substrate with highly deformable properties. There are examples of thermoelectric materials prepared in fiber form and optimized for thermoelectric efficiency by virtue of a unique textile architecture that combines the advantages of material flexibility and structural flexibility. These flexible thermoelectric materials retain a high degree of flexibility and permeability, and have the advantage of manufacturing cost-effective and reliable high-volume products through industrial processes at room temperature. This property makes the material show a great application potential in flexible electronic devices.This review systematically summarizes the related research progress and points out the synergistic enhancement of flexibility, stability, and output performance during the preparation and design optimization process. This provides a useful reference and guidance for the preparation and design optimization of flexible thermoelectric devices.Summary and prospectsAlthough flexible thermoelectric technology has made remarkable progress in waste heat power generation, wearable devices and smart textiles, having a great application potential, there are still a series of problems in practical application. Therefore, a more systematic and comprehensive study is needed. The output of flexible substrate thermoelectric devices depends on the heat harvesting area and temperature gradient, and can produce power densities ranging from a few microwatts per square centimeter to a few milliwatts per square centimeter. However, the interface thermal resistance of low-dimensional flexible devices will lead to a low conversion efficiency, and its preparation technology is still limited to a large-scale application. Major challenges for fiber thermoelectric devices include building larger temperature gradients in the direction of heat flow, designing scalable architectures, improving mechanical stability, and enhancing comfort. In addition, the development of advanced preparation technologies such as magnetron sputtering and pulsed laser deposition will also achieve a large-scale production. Also, the combination of boost and energy storage technologies, such as efficient DC-DC converters and new energy storage systems, will effectively solve the problem of low output voltage of thermoelectric devices, so that they can directly drive low-power sensors or communication modules. Finally, with the further development of interface stability and flexible design of flexible substrate thermoelectric devices, flexible thermoelectric devices will be more widely used in wearable technology, smart textiles and some related fields.