Biosensor technology is a powerful alternative to conventional techniques, harnessing the specificity and sensitivity of biological systems in small, low cost devices. Despite the promising biosensors developed in research laboratories, there are not many reports of applications in agricultural monitoring. The authors review biosensor technology and discuss the different bio-receptor systems and methods of transduction. The difference between a biosensor and a truly integrated biosensor system are defined and the main reasons for the slow technology transfer of biosensors to the marketplace are reported. Biosensor research and development has been directed mainly towards health care, environmental applications and the food industry. The most commercially important application is the hand-held glucose meter used by diaberics. The agricultural/veterinary testing market has seen a number of diagnostic tests but no true biosensor systems have made an impact. The need for fast, on-line and accurate sensing opens up opportunities for biosensors in many different agricultural areas -in situ analysis of pollutants in crops and soils, detection and identification of diseases in crops and livestock, on-line measurements of important food processing parameters, monitoring animal fertility and screening therapeutic drugs in veterinary testing. Further challenges in the commercial development of biosensors are also addressed.