Abstract Vibrations created by drag bits can cause premature failure of drillstring components and fracture cutters. Unfortunately, there has been no theory that adequately explains how drag bits excite drillstrings. Our paper shows that these vibrations are self excited by the interaction of drillstring, bit, and formation. Results show that drag bit design and bit rotational speed dictate conditions of dynamic instability. In general, large drag bits are unstable at low rotational speeds.