This paper presents the general principles of various biosensor systems, reviews current biosensor technologies for agricultural and environmental monitoring, and discusses their opportunities and challenges. Advances in biosensor technologies could provide a useful analytical tools for agricultural monitoring, particularly due to their rapid response, relatively low operational cost and portability for field/farm application. The promise, demonstrated by various examples of biosensor technologies, is very appealing. However, there are still many hurdles to bring commercial agricultural biosensors into real practice.