Bio-chips need active surfaces for sensing, actuation, synthesis and analysis. Thus, they should be realized with materials, widely suppressing corrosion and hydrolysis in combination with conducting, semiconducting and insulating properties, biocompatibility and chemical inertness. An ideal candidate is diamond. Two forms are considered: single crystal and nanocrystalline diamond, the later also containing non-diamond phases and graphic grain boundaries. Their surface properties and functionalization are discussed together with their electrode, ISFET and cell attachment and related neuron activity characteristics. Furthermore an outlook is given for system approaches.