This review, based on Theophilus Redwood Medal and Award lectures delivered in 2012, provides a comprehensive overview of the field of biosensors. The biosensors industry is now worth billions of US dollars, with applications in medicine, pharma, food and process control, environmental monitoring, defense, and security. The most significant market is driven by medical diagnostics, particularly glucose sensors for people with diabetes. Electrochemical biosensors dominate the field, focusing on metabolite monitoring, while optical techniques are used for bioaffinity monitoring. The emergence of personalized medicine is likely to impact biosensors significantly. Semi-synthetic and synthetic receptors are making sensors more robust and versatile, while nanomaterials enhance sensitivity and convenience. The review also discusses the potential for inexpensive, wearable, and less-invasive sensors to support health maintenance and care. The success of biosensors is attributed to their ability to combine biological sensitivity with physicochemical transducers, making complex bioanalytical measurements simple and easy to use. The review highlights key technological drivers, applications, and future potentials, emphasizing the importance of biosensors in various fields.This review, based on Theophilus Redwood Medal and Award lectures delivered in 2012, provides a comprehensive overview of the field of biosensors. The biosensors industry is now worth billions of US dollars, with applications in medicine, pharma, food and process control, environmental monitoring, defense, and security. The most significant market is driven by medical diagnostics, particularly glucose sensors for people with diabetes. Electrochemical biosensors dominate the field, focusing on metabolite monitoring, while optical techniques are used for bioaffinity monitoring. The emergence of personalized medicine is likely to impact biosensors significantly. Semi-synthetic and synthetic receptors are making sensors more robust and versatile, while nanomaterials enhance sensitivity and convenience. The review also discusses the potential for inexpensive, wearable, and less-invasive sensors to support health maintenance and care. The success of biosensors is attributed to their ability to combine biological sensitivity with physicochemical transducers, making complex bioanalytical measurements simple and easy to use. The review highlights key technological drivers, applications, and future potentials, emphasizing the importance of biosensors in various fields.