Thermoacoustic instability was investigated and controlled in an experimental low-emission swirl stabilized combustor, in which the acoustic boundary conditions were modified to obtain combustion instability. Several axisymmetric and helical unstable modes were identified for fully premixed and partially premixed/diffusion combustion. These unstable modes were associated with flow instabilities related to the recirculation region on the combustor axis and shear layer instabilities at the sudden expansion (dump plane). The spatial locations of the intense combustion regions associated with the different unstable modes were visualized by phase locked images of OH chemiluminescence. The axisymmetric mode showed large variation of the heat release during one cycle, while the helical modes showed variations in the radial location of maximal heat release. A closed loop active control system was employed to suppress the thermoacoustic pressure oscillations and to reduce NOx emissions. Microphone and OH emission sensors were utilized to monitor the combustion process and provide input to the control system. Acoustic actuation was utilized to modulate the air flow to affect the mixing process and the combustion. Suppression levels of up to 5 dB in the pressure oscillations and a concomitant reduction of NO, emissions were obtained using an acoustic power of less than 0.002% of the combustion power. The microphone based controller was slightly more efficient than the OH-based controller. This was due to the reduced coherence of the combusting, large-scale structures which resulted in a deterioration of the OH signal when the controller became effective.