With the proliferation of microelectronic devices, the need for portable power supplies is evidently increasing. Possible candidates for micro energy storage devices are Li-ion batteries and supercapacitors. Among them, the flexible solid-state supercapacitors combined with exceptionally long cycle life, high power density, environmental friendliness, safety, flexibility and stability, afford a very promising option for energy storage applications. This paper reviews flexible solid-state electrochemical supercapacitors and the performance metrics. A better practice by calculating released energy to evaluate material and device performance is proposed. In addition, an overview is given of the electrolyte and various electrode materials that are suitable for flexible solid-state supercapacitors. The review summarizes the recent research focusing on novel configurations of flexible solid-state supercapacitors for energy storage and applications, such as freestanding, asymmetric, interdigitated, and fiber-based supercapacitors. Lastly, some discussions on future research are presented.