A review of a tunable active bandpass filter developed in our research group is given here. Presented first is the basic structure of an end-coupled microstrip-line bandpass filter. Next discussed is the concept of using coupled negative resistance to compensate the loss of tank circuit. Then, the method of using varactor diode or mesfet varactor to tune the center frequency of the passband is discussed. The capability of optical-control of mesfet varactor and the concept of using mesfets as three-terminal varactors are also discussed.