An alternative to vertical tails, wing-tip fins, a type of winglet, provide yaw stability and control and some operational and maintenance advantages. Winglets reduce wing-induced drag but may reduce flutter speed, requiring wing stiffening and a structural weight penalty. Whereas the number of successful aircraft with winglets suggests that many flutter studies have been done, few of these studies appear in the archival literature. This paper investigates the effect of tip-fin surface geometry on flutter as well as on directional stability. The study includes the effects of tip-fin cant angle with respect to the primary surface and fin size with respect to the wing itself. Two idealized models, a simple four degree-of-freedom Rayleigh–Ritz model and a high-fidelity finite element model, illustrate special features of tip-fin aeroelasticity. Results indicate that the interaction between tip fins and the wing flexibility usually leads to reduced flutter speeds but can also create phenomena such as mode swi...