This discussion summarizes the interaction of refractory periods and upper rate behaviors in modern dual-chamber demand (DDD) devices, the data regarding and nine events initiating VA conduction and engineering solutions proposed and/or implemented to address the problem of pacemaker-mediated tachycardia (PMT). Among the causes of PMT are premature atrial depolarization, loss of atrial capture, a return to the demand mode after asynchronous magnet mode pacing, programming from a mode that does not guarantee AV synchrony to a mode in which atrial tracking can occur, noise, certain situations involving Wenckebach behavior, loss of sensing, and the inability of a rate-smoothing algorithm to allow a rapid change in ventricular rate. Engineering solutions to prevent the occurrence of PMT include a programmable postventricular atrial refractory period (PVARP), differential AV delay, adaptive AV delay, and the ability to discriminate between P waves of atrial origin and those resulting from retrograde conduction from the ventricle. Features such as the ability to lengthen the PVARP for one cycle after exiting the magnet or noise reversion modes or programming to a new mode, lengthen the PVARP for a single cycle following a PVC or revert to DVI pacing for one cycle following a PVC have been developed to recognize initiating events. A third solution, a tachycardia termination algorithm, can recognize and terminate PMT; varying the AV delay to determine whether P waves move in a corresponding manner and using a metabolic sensor to confirm the need for a fast heart rate are other possibilities in the detection of PMT. Diagnostic data features may also be used to evaluate the appropriateness of programmed settings. This discussion concludes that PMT is no longer a significant clinical entity when more advanced DDD pacemakers are utilized.