As an electrode material, carbon has versatile properties that originate from its structural polymorphism, chemical stability, rich surface chemistry and strong carbon-carbon bonds. This chapter discusses the electrocatalytical properties of carbon nanomaterials. Moreover, typical sensing mechanisms, direct detection and biosensors based on catalytic and affinity-based bioreceptors are discussed with examples of the usage of carbonaceous materials. Particular focus is on the challenges of electrochemical measurements in the biological environment, including (i) sensitivity, (ii) selectivity and (iii) stability and (iv) biocompatibility of the sensor.