The introduction to the chapter on biosensors provides a comprehensive overview of the field, highlighting its widespread applications and technological advancements. Biosensors are devices that measure biological or chemical reactions by generating signals proportional to the concentration of an analyte. They are used in various fields, including biomedical diagnosis, environmental monitoring, food control, drug discovery, forensics, and biomedical research. The chapter covers the historical background of biosensors, from early demonstrations of pH measurement to the development of the first commercial biosensor in 1975. It also discusses key characteristics of biosensors, such as selectivity, reproducibility, stability, sensitivity, and linearity, which are crucial for their performance and reliability. The applications of biosensors are diverse, ranging from single-shot analysis tools like pregnancy tests to long-term monitoring systems for pollution and disease detection. The integration of nanotechnology has significantly advanced biosensor performance, enabling miniaturization, improved signal-to-noise ratios, and single-molecule detection. Graphene and graphene oxide have opened new frontiers in biosensing, allowing for lower limits of detection and enhanced stability. The impact of biosensors on society is evident, with their use in healthcare, homeland defense, agriculture, and environmental monitoring. However, challenges in commercializing biosensors, such as regulatory issues and the need for interdisciplinary collaboration, persist. Despite these challenges, the rapid development in biosensing technologies, driven by advancements in miniaturization, novel bio-recognition molecules, and nanomaterials, is expected to continue, leading to more advanced and widely applicable biosensor devices.The introduction to the chapter on biosensors provides a comprehensive overview of the field, highlighting its widespread applications and technological advancements. Biosensors are devices that measure biological or chemical reactions by generating signals proportional to the concentration of an analyte. They are used in various fields, including biomedical diagnosis, environmental monitoring, food control, drug discovery, forensics, and biomedical research. The chapter covers the historical background of biosensors, from early demonstrations of pH measurement to the development of the first commercial biosensor in 1975. It also discusses key characteristics of biosensors, such as selectivity, reproducibility, stability, sensitivity, and linearity, which are crucial for their performance and reliability. The applications of biosensors are diverse, ranging from single-shot analysis tools like pregnancy tests to long-term monitoring systems for pollution and disease detection. The integration of nanotechnology has significantly advanced biosensor performance, enabling miniaturization, improved signal-to-noise ratios, and single-molecule detection. Graphene and graphene oxide have opened new frontiers in biosensing, allowing for lower limits of detection and enhanced stability. The impact of biosensors on society is evident, with their use in healthcare, homeland defense, agriculture, and environmental monitoring. However, challenges in commercializing biosensors, such as regulatory issues and the need for interdisciplinary collaboration, persist. Despite these challenges, the rapid development in biosensing technologies, driven by advancements in miniaturization, novel bio-recognition molecules, and nanomaterials, is expected to continue, leading to more advanced and widely applicable biosensor devices.