Active vibration isolation systems using negative stiffness were presented. Connecting a negative-stiffness suspension with a normal spring in series can generate infinite stiffness against disturbances acting on the isolation table. This property is maintained even if a suspension to support the weight of the isolation table is introduced in parallel with the serial combination. The principles and fundamental characteristics of the systems were described in a analytical form together with experimental apparatuses developed for experimental study and experimental results. It was experimentally confirmed that combining a negative-stiffness suspension with a normal spring in series generates high stiffness against static direct disturbance acting on an isolation table. Vibration isolation using negative stiffness is a quite unique approach. Very high (theoretically infinite) stiffness to direct vibration is achieved with low-cost sensors while system vibration transmitted from the ground is reduced. Since negative stiffness can be achieved by any linear actuator including electromagnetic actuator (zero-power magnetic suspension), various types of system are possible. Active research and development has been and will be continued to industrial applications (Mizuno et al., 2007 & 2008).