Biosensors exploit the specific binding between recognition molecule on the biosensor surface and target molecule in analyte and are used in the detection of specific biomolecules such as protein, DNA, cell, virus, etc., with a view towards developing analytical devices. Recently, application field of biosensors have been expanding from diagnosis to biodefense because they can basically serve as high performance devices. This review describes the basic information of biosensors including definition, classification, and operational principle. Moreover, we introduce micro/nano technology-based biosensors with better detection performance than traditional method and their application examples.