Organic electrochemical transistors (OECTs) use ion injection from an electrolyte into an organic semiconductor film to enable advances in biological interfacing, printed logic circuits, and neuromorphic devices. Their key feature is the coupling of electronic and ionic charges within the organic film. This review discusses OECT operation mechanisms, materials, form factors, fabrication technologies, and applications, while critically examining future research and development. OECTs were developed in the 1980s and operate by injecting ions from an electrolyte into the organic film, changing its doping state and conductivity. The gate voltage controls ion injection, while the drain voltage induces a current proportional to the number of mobile charges in the channel. OECTs can function as switches or amplifiers, with high transconductance values (up to mS for micron-scale devices) due to their volumetric response. A common material for OECTs is PEDOT:PSS, a conducting polymer with high hole conductivity and good electrochemical stability in aqueous electrolytes. OECTs based on PEDOT:PSS operate in depletion mode, where a hole current flows in the channel without a gate voltage, and in accumulation mode, where a negative gate voltage causes anion injection and hole accumulation. OECTs have a unique advantage in volumetric gating, allowing large modulations in drain current with low gate voltages, making them efficient switches and powerful amplifiers. OECTs have a high transconductance but relatively slow operation, limited by the ionic circuit or electronic circuit. The response time is determined by the product of the electrolyte resistance and the channel capacitance. OECTs with liquid electrolytes can achieve response times as fast as tens of microseconds, suitable for many biosensor applications. However, gel or solid electrolytes slow down OECTs, though they still offer potential for printed electronics. OECTs are electrolyte-gated devices, with the gate electrode's geometry and material affecting the voltage drop across the channel. A large gate electrode is preferred for efficient gating, while a small gate electrode may be suitable for sensing applications where the channel acts as the transducer. The nature and ion concentration of the electrolyte influence the response time, as electrolyte conductivity determines the ionic circuit resistance. OECTs are typically made of conducting polymers like PEDOT:PSS, which have high electronic conductivity and good compatibility with biological environments. However, PEDOT:PSS has limitations, including a complex structure, limited volumetric capacitance, and high Young's modulus, which may be unsuitable for bioelectronics. New materials, such as less acidic polyanions and conjugated polyelectrolytes, are being explored to improve performance and biofunctionalization. OECTs have been used in various applications, including bioelectronics, biosensors, printed circuits, and neuromorphic devices. They are particularly useful in bioelectronics for interfacing with electrically active tissues andOrganic electrochemical transistors (OECTs) use ion injection from an electrolyte into an organic semiconductor film to enable advances in biological interfacing, printed logic circuits, and neuromorphic devices. Their key feature is the coupling of electronic and ionic charges within the organic film. This review discusses OECT operation mechanisms, materials, form factors, fabrication technologies, and applications, while critically examining future research and development. OECTs were developed in the 1980s and operate by injecting ions from an electrolyte into the organic film, changing its doping state and conductivity. The gate voltage controls ion injection, while the drain voltage induces a current proportional to the number of mobile charges in the channel. OECTs can function as switches or amplifiers, with high transconductance values (up to mS for micron-scale devices) due to their volumetric response. A common material for OECTs is PEDOT:PSS, a conducting polymer with high hole conductivity and good electrochemical stability in aqueous electrolytes. OECTs based on PEDOT:PSS operate in depletion mode, where a hole current flows in the channel without a gate voltage, and in accumulation mode, where a negative gate voltage causes anion injection and hole accumulation. OECTs have a unique advantage in volumetric gating, allowing large modulations in drain current with low gate voltages, making them efficient switches and powerful amplifiers. OECTs have a high transconductance but relatively slow operation, limited by the ionic circuit or electronic circuit. The response time is determined by the product of the electrolyte resistance and the channel capacitance. OECTs with liquid electrolytes can achieve response times as fast as tens of microseconds, suitable for many biosensor applications. However, gel or solid electrolytes slow down OECTs, though they still offer potential for printed electronics. OECTs are electrolyte-gated devices, with the gate electrode's geometry and material affecting the voltage drop across the channel. A large gate electrode is preferred for efficient gating, while a small gate electrode may be suitable for sensing applications where the channel acts as the transducer. The nature and ion concentration of the electrolyte influence the response time, as electrolyte conductivity determines the ionic circuit resistance. OECTs are typically made of conducting polymers like PEDOT:PSS, which have high electronic conductivity and good compatibility with biological environments. However, PEDOT:PSS has limitations, including a complex structure, limited volumetric capacitance, and high Young's modulus, which may be unsuitable for bioelectronics. New materials, such as less acidic polyanions and conjugated polyelectrolytes, are being explored to improve performance and biofunctionalization. OECTs have been used in various applications, including bioelectronics, biosensors, printed circuits, and neuromorphic devices. They are particularly useful in bioelectronics for interfacing with electrically active tissues and