Impedance biosensors are a class of electrical biosensors that are promising for point-of-care diagnostics and other applications due to their low cost, ease of miniaturization, and label-free operation. These sensors detect changes in surface impedance when target molecules bind to immobilized probes. The review discusses the challenges unique to impedance readout, such as the need for sensitive readout methods and selectivity in complex samples. It also covers the general principles of label-free affinity impedance biosensors, including the motivation for studying them, the concepts of affinity and sensor, and the practical issues in measuring electrochemical impedance. The review highlights the importance of probe immobilization chemistries and minimizing nonspecific binding to improve performance. It also explores the various mechanisms that cause impedance changes upon target binding, such as displacement of water, changes in dielectric properties, and increased resistance due to electrostatic repulsion. The response curve, which relates the sensor output to target concentration, is discussed, along with the observed logarithmic relationship in low concentrations. The review concludes by summarizing the current status of affinity-based impedance biosensors and identifying areas for future research.Impedance biosensors are a class of electrical biosensors that are promising for point-of-care diagnostics and other applications due to their low cost, ease of miniaturization, and label-free operation. These sensors detect changes in surface impedance when target molecules bind to immobilized probes. The review discusses the challenges unique to impedance readout, such as the need for sensitive readout methods and selectivity in complex samples. It also covers the general principles of label-free affinity impedance biosensors, including the motivation for studying them, the concepts of affinity and sensor, and the practical issues in measuring electrochemical impedance. The review highlights the importance of probe immobilization chemistries and minimizing nonspecific binding to improve performance. It also explores the various mechanisms that cause impedance changes upon target binding, such as displacement of water, changes in dielectric properties, and increased resistance due to electrostatic repulsion. The response curve, which relates the sensor output to target concentration, is discussed, along with the observed logarithmic relationship in low concentrations. The review concludes by summarizing the current status of affinity-based impedance biosensors and identifying areas for future research.