There is a great need for an accurate and rapid analytical technique to detect hazardous chemical/biological substances. Various detection methodologies, such as optical, electrochemical, surface plasmon resonance, and magnetic resonance techniques, have shown excellent sensing performance toward target molecules. The observation of signal changes based on nanoelectronics enables highly sensitive and selective recognition and real-time responses. Among the many functional materials used as signal transducers, graphene, a carbon allotrope with an atomic-scale two-dimensional planar structure, is of special interest. Graphene possesses excellent electrical and electronic properties, such as high carrier mobility and capacity, ambipolar field effect, and highly tunable conductance. It is used broadly in electronic, optoelectronic, energy, and environmental applications. In particular, because graphene has a large surface-to-volume ratio, extraordinary carrier mobility, and high compatibility, nanoscale graphene sensors are very promising. Herein, we introduce state-of-the-art biosensor technologies based on various types of graphene, especially field-effect-transistor-type and electrochemical biosensors.