The fabrication and characterization of resonant silicon accelerometers, made in bulk micromachining technology, is presented. The devices consist of a silicon mass, coupled axially to a strain-sensitive vibrating silicon beam. The beam is driven electrothermally and sensed piezoresistively by means of implanted piezoresistors. Two different accelerometer types are shown, differing in the complexity of the respective fabrication processes and in performances. Closed-loop operation of the devices is demonstrated. Also in the closed loop, static and dynamic measurements of prototypes have been performed. The sensor types presented are compared, and the resonant acceleration sensor concept is discussed.