A key challenge of the current research in nanoelectronics is the realization of biomolecular devices. The use of electron-transfer proteins, such as the blue copper protein, azurin (Az), is particularly attractive because of its natural redox properties and self-assembly capability. This chapter dis- cusses results of fabrication, characterization, and modeling of devices based on this redox protein. The prototype of biomolecular devices operate in the solid state and in air. The charge transfer process in protein devices can be engineered by using proteins with different redox centers (metal atoms) and by controlling their orientation in the solid state through different immobilization methods. A biomolecular electron rectifier has been demonstrated by interconnecting two gold nanoelectrodes with an Az monolayer immobilized on SiO2. The device exhibits a clear rectifying behavior with discrete current steps in the positive wing of the current— voltage (I-V) curve, which is ascribed to resonant tunneling through the redox active center. On the basis of these results, an Az-based three- terminal device has been designed. The three-terminal device exhibits an ambipolar behavior as a function of the gate bias, thus opening the way to the implementation of a new generation of logic architectures. This peculiar characteristic allows the implementation of a fully integrated nanoscopic logic gate.