Organic electrochemical transistors (OECTs) have emerged as a promising platform for biomarker detection due to their high sensitivity, low operating voltage, and compatibility with wearable and implantable devices. This review discusses the working principles of OECTs, their performance in detecting various biomarkers, and their recent applications in wearable and implantable biosensing. OECTs can convert biological signals into electrical signals and amplify them in situ, making them highly sensitive for detection. They are particularly useful for detecting nucleic acids, proteins, metabolites, neurotransmitters, hormones, cells, bacteria, viruses, and electrophysiological signals. OECTs offer advantages such as simple device structure, low working voltages, high transconductance, biocompatibility, and flexibility, making them suitable for a wide range of biomedical applications. OECTs are composed of three electrodes, an organic semiconductor channel, and an electrolyte that connects the gate and channel. The gate electrode can be integrated with the channel or separated, allowing for convenient modification of biomolecules on the surface. The device performance is primarily determined by the geometric dimensions and the figures of merit of the channel materials. OECTs can operate at very low voltages, which is beneficial for preserving the structural integrity of biological molecules and minimizing potential harm to cell analytes during testing. The transconductance of OECTs is a critical parameter that reflects their ability to efficiently amplify electrical signals, and they have been shown to exceed those of state-of-the-art metal oxide semiconductor FETs and FETs based on two-dimensional materials. OECTs are biocompatible and can be used in cell detection and in vivo implant testing. They are composed of thin gold or platinum films, organic semiconductor films, and substrates. Conductive polymers, such as PEDOT:PSS, are biocompatible and can interact with biological systems, stimulate cell growth, and promote tissue regeneration. OECTs are flexible and can be fabricated on flexible substrates such as plastics, paper, and textiles, making them suitable for wearable and implantable medical devices. OECTs have been used for the detection of various biomarkers, including genomic, proteomic, metabolic, and electrophysiological biomarkers. Genomic biomarkers are variations in DNA or RNA that can be measured as indicators of disease or response to treatment. Proteomic biomarkers are proteins that can be used to diagnose diseases, including cancer, cardiovascular disease, and neurological disorders. Metabolic biomarkers are small molecules that can be detected through self-redox reactions or enzyme sensing methods. Hormones and neurotransmitters are chemical messengers that have crucial roles in intercellular communication and can be detected using OECTs. OECTs have been used for the detection of various biomarkers, including microRNA, proteins, metabolites, hormones, and neurotransmitters. They have been shown to offer high sensitivity and low detection limits, making them suitable for early disease diagnosisOrganic electrochemical transistors (OECTs) have emerged as a promising platform for biomarker detection due to their high sensitivity, low operating voltage, and compatibility with wearable and implantable devices. This review discusses the working principles of OECTs, their performance in detecting various biomarkers, and their recent applications in wearable and implantable biosensing. OECTs can convert biological signals into electrical signals and amplify them in situ, making them highly sensitive for detection. They are particularly useful for detecting nucleic acids, proteins, metabolites, neurotransmitters, hormones, cells, bacteria, viruses, and electrophysiological signals. OECTs offer advantages such as simple device structure, low working voltages, high transconductance, biocompatibility, and flexibility, making them suitable for a wide range of biomedical applications. OECTs are composed of three electrodes, an organic semiconductor channel, and an electrolyte that connects the gate and channel. The gate electrode can be integrated with the channel or separated, allowing for convenient modification of biomolecules on the surface. The device performance is primarily determined by the geometric dimensions and the figures of merit of the channel materials. OECTs can operate at very low voltages, which is beneficial for preserving the structural integrity of biological molecules and minimizing potential harm to cell analytes during testing. The transconductance of OECTs is a critical parameter that reflects their ability to efficiently amplify electrical signals, and they have been shown to exceed those of state-of-the-art metal oxide semiconductor FETs and FETs based on two-dimensional materials. OECTs are biocompatible and can be used in cell detection and in vivo implant testing. They are composed of thin gold or platinum films, organic semiconductor films, and substrates. Conductive polymers, such as PEDOT:PSS, are biocompatible and can interact with biological systems, stimulate cell growth, and promote tissue regeneration. OECTs are flexible and can be fabricated on flexible substrates such as plastics, paper, and textiles, making them suitable for wearable and implantable medical devices. OECTs have been used for the detection of various biomarkers, including genomic, proteomic, metabolic, and electrophysiological biomarkers. Genomic biomarkers are variations in DNA or RNA that can be measured as indicators of disease or response to treatment. Proteomic biomarkers are proteins that can be used to diagnose diseases, including cancer, cardiovascular disease, and neurological disorders. Metabolic biomarkers are small molecules that can be detected through self-redox reactions or enzyme sensing methods. Hormones and neurotransmitters are chemical messengers that have crucial roles in intercellular communication and can be detected using OECTs. OECTs have been used for the detection of various biomarkers, including microRNA, proteins, metabolites, hormones, and neurotransmitters. They have been shown to offer high sensitivity and low detection limits, making them suitable for early disease diagnosis