We consider a second-order equation of Duffing type.
Bounds for the derivative of the restoring force are given which
ensure the existence and uniqueness of a periodic solution.
Furthermore, the unique periodic solution is asymptotically stable
with sharp rate of exponential decay. In particular, for a restoring
term independent of the variable $t$, a necessary and sufficient
condition is obtained which guarantees the existence and
uniqueness of a periodic solution that is stable.