Wellbore resonance phenomena are the basis fora new acoustic logging method, the Stimulated Mode Acoustic Log (SMAL). The SMAL method stimulates a particular mode of wellbore vibration by exciting a resonance for that mode. Specific resonances are excited by driving an acoustic transmitter with a short tone burst of appropriate frequency. SMAL benefits include signal enhancement under adverse conditions, avoiding dispersion effects, propagating formation waves through cased and slotted liner completions, generating steady-state phase and amplitude data, and providing formation Q logs. All of these applications have arisen from studies of wellbore resonance phenomena through well logging and laboratory experiments. One means of study has been the wellbore excitation log, which shows the foot-by foot response of the wellbore to swept-frequency excitation across the acoustic band. Excitation logs show that resonances are sensitive to lithology and formation saturation conditions. Additional insight has been provided by generating standing waves under resonance conditions. This approach has led to a means of estimating a formation Q from the decay of standing tube waves. Q data, generated in this way, are consistent with laboratory data for core and quarry samples. A Q log, run through the gas, oil, and water legs of a thick Gulf Coast sand, is consistent with lab data for cores. Laboratory experiments in thick-wall pipe models have been another means of study. These field and laboratory investigations indicate that the combination of wellbore and logging tool behave somewhat like a resonance cavity for pressure waves in the wellbore fluid. Tool elements, especially the two ends, act as reflectors of fluid wave energy, giving rise to resonances and standing wave phenomena. The resonance characteristics of this cavity are dominated by the properties of the formation wall material and the coupling between the fluid and borehole wall. Other factors, such as fluid column and source resonances, play secondary roles. It is the strong dependence of resonances on formation properties that makes SMAL a valuable logging concept.