The chapter discusses the advancements and applications of Organic Electrochemical Transistors (OECTs) in biomarker detection. OECTs have gained attention due to their high sensitivity, low detection limits, and versatility in detecting various biomarkers, including nucleic acids, proteins, metabolites, neurotransmitters, hormones, cells, bacteria, viruses, and electrophysiological signals. The introduction highlights the importance of biomarker detection in healthcare, emphasizing the role of OECTs in early disease diagnosis and personalized medicine. The working principles of OECTs, their device structure, and key features such as low working voltages, high transconductance, biocompatibility, and flexibility are detailed. The chapter also explores the detection of specific biomarker categories, including genomic, proteomic, metabolic, and neurochemical biomarkers, with examples of label-free and labeled detection methods. OECTs' ability to interface with biological systems and convert biological signals into electrical signals is highlighted, along with their potential for wearable and implantable medical devices. The chapter concludes by discussing the future perspectives and challenges in the development of OECT-based biomarker sensors.The chapter discusses the advancements and applications of Organic Electrochemical Transistors (OECTs) in biomarker detection. OECTs have gained attention due to their high sensitivity, low detection limits, and versatility in detecting various biomarkers, including nucleic acids, proteins, metabolites, neurotransmitters, hormones, cells, bacteria, viruses, and electrophysiological signals. The introduction highlights the importance of biomarker detection in healthcare, emphasizing the role of OECTs in early disease diagnosis and personalized medicine. The working principles of OECTs, their device structure, and key features such as low working voltages, high transconductance, biocompatibility, and flexibility are detailed. The chapter also explores the detection of specific biomarker categories, including genomic, proteomic, metabolic, and neurochemical biomarkers, with examples of label-free and labeled detection methods. OECTs' ability to interface with biological systems and convert biological signals into electrical signals is highlighted, along with their potential for wearable and implantable medical devices. The chapter concludes by discussing the future perspectives and challenges in the development of OECT-based biomarker sensors.