Recent advances in digital signal processing and actuation technology have opened the door to applying active control to reducing mechanical vibrations and acoustic noise. Because these techniques work best at lower frequency, they complement conventional passive damping techniques, which are most effective at higher frequencies. Engineers in Lucent Technologies' Advanced Technology Systems have been developing active vibration and noise control systems for the Defense Advanced Research Project Agency (DARPA) and the Department of Defense (DoD) since 1988. This paper describes the design methodology of active control, the class of problems that responds best to this solution, and the complementary nature of active and passive vibration reduction strategies. It also gives a guide for when, where, and how to use active control to solve practical vibration problems.