The NOx emissions of heavy duty gas turbines were significantly reduced by introducing lean premixed combustion. These flames are known to be prone to combustion instabilities. In this paper, investigations of a single model gas turbine burner are presented with focus on thermo-acoustic eigenmodes of the combustor and the resulting interaction between periodic flow field oscillations and flame front fluctuations. A numerical anal. of the eigenmodes of the combustor rig was performed and compared to pressure measurements. By the numerical anal. Helmholtz mode pressure oscillations were predicted to be present in the air plenum upstream the burner and in the combustion chamber. These oscillation modes were confirmed by the anal. of the pressure signals taken at different positions within the combustor rig. Two major achievements were proved by exptl. evidence: First, the presence of an acoustic eigenmode between plenum and combustor may trigger the combustion instabilities. Due to a phase lag between the pressure oscillations in the air plenum and those in the combustion chamber, fluctuations of air flow through the burner are induced, which cause fluctuations of the equivalence ratio at the burner exit plane. As result, fluctuations of the heat release rate within the flame are established, which interfere with the acoustics of the rig and, thus, unstable combustion results. Second, the amplitude of the combustion oscillations does not increase smoothly when increasing the air fuel ratio. Two different flow field patterns were identified in the combustor depending on the amplitude of the oscillations. At low oscillation levels weak and locally confined velocity fluctuations were obsd. in the main reaction zone which do not have any significant impact on the flame. By increasing the equivalence ratio high oscillation levels occur and a pumping motion of the flow field in axial direction was identified which causes periodic heat release fluctuations and, hence, a strong disturbance of the flame.