  Hardware injections are simulated gravitational-wave signals added to the
Laser Interferometer Gravitational-wave Observatory (LIGO). The detectors' test
masses are physically displaced by an actuator in order to simulate the effects
of a gravitational wave. The simulated signal initiates a control-system
response which mimics that of a true gravitational wave. This provides an
end-to-end test of LIGO's ability to observe gravitational waves. The
gravitational-wave analyses used to detect and characterize signals are
exercised with hardware injections. By looking for discrepancies between the
injected and recovered signals, we are able to characterize the performance of
analyses and the coupling of instrumental subsystems to the detectors' output
channels. This paper describes the hardware injection system and the recovery
of injected signals representing binary black hole mergers, a stochastic
gravitational wave background, spinning neutron stars, and sine-Gaussians.
