organic electronic devices have attracted substantial attention. [18][19][20][21][22][23][24] However, many issues still exist. For instance, the flexible substrates can be easily bent and thus cause damage to the devices or affect their electrical performance. Moreover, the flexible substrates are sensitive to operating conditions and are unstable during longterm operation. [25][26][27][28][29] Therefore, many research groups have dedicated their efforts to study and improve the flexibility and stretchability of organic electronic devices. [30][31][32][33] In recent years, organic three-terminal synaptic transistors have evolved from two-terminal devices to avoid the problem of crosstalk between adjacent cells. [34] This is an evolutionary structural design to improve the function of organic synaptic devices. In addition, the organic three-terminal synaptic transistors can be designed to form a highly interconnected neural network system. [35] At the same time, the organic three-terminal synaptic transistors can concurrently receive and read stimuli, which is conducive to the design of multiple input devices. [36] Currently, the threeterminal artificial synapses devices based on flexible organic transistors are considered to be the representative structures of biomimetic devices, which can be used to simulate the plasticity of biological synapses. [37,38] Compared with the traditional inorganic synaptic devices, the flexible organic synaptic transistors (FOSTs) can be used to simulate the plasticity of the human brain with a simpler structure and lower manufacturing cost. Therefore, a FOST is a promising component of future organic neuromorphic systems. [39][40][41][42][43][44][45][46] To date, the FOSTs have been widely used in wearable electronic devices and intelligent e-skins. [47][48][49] In 2018, a flexible artificial afferent nerve was reported by Xu et al. [47] representing significant progress toward the development of intelligent e-skin and soft robotics. This paper reviews the recent progress in the development of FOSTs and their applications in neuromorphic systems. Generally, the FOSTs are divided into four categories: flexible organic floating-gate synaptic transistors (FO-FGSTs), flexible organic ferroelectric-gate synaptic transistors (FO-FeGSTs), flexible organic electrolyte-gate synaptic transistors (FO-EGSTs), and flexible organic optoelectronic synaptic transistors (FO-OSTs). First, a brief introduction of the device structure mostly used in the FOSTs is provided. Then, the selection of substrate materials, gate dielectric materials, organic channel materials, and electrode materials in the FOSTs is summarized. Next, the major advances of these FOSTs and their potential applications are reviewed and discussed. Lastly, the summary, outlook, and In recent years, flexible organic synaptic transistors have attracted considerable attention due to their flexibility, biocompatibility, easy processability, and reduced complexity. Flexible organic synaptic transistors have functions and structures sim...