This chapter presents the main categories of biosensors, tracing their evolution from the earliest electrochemical devices to specialized immunosensors that define modern diagnostic practice. Section 2.1 introduces electrochemical biosensors, the foundation of contemporary biosensing, covering their historical development, operational principles, and clinical significance. Section 2.2 turns to optical biosensors, where light-based transduction enables exceptional sensitivity and versatility across research and medicine. Section 2.3 focuses on impedance biosensors, highlighting their ability to characterize cellular and molecular interactions without labels, a key advantage for real-time monitoring. Section 2.4 explores acoustic and piezoelectric biosensors, where mechanical waves serve as the interface between biological recognition and physical response. Section 2.5 examines electrical and field-effect biosensors, tracing the path from early semiconductor sensors to advanced BioFET architectures integrating nanoscale materials and wearable electronics. Finally, Section 2.6 discusses immunosensors, which merge immunochemical specificity with cutting-edge signal processing to achieve precision diagnostics. Together, these sections chart the technological and conceptual diversity that defines the biosensor field, illustrating how different transduction strategies converge toward a shared goal: transforming biological information into actionable data.