Systems and methods for dynamically adjusting the fit of a wearable electronic device are disclosed. In many embodiments, a tensioner associated with a wearable electronic device can control one or more actuators that are mechanically coupled to either the housing or to a band attached to the wearable electronic device. In one example, in response to a signal to increase the tightness of the band, the tensioner can cause the actuator(s) to increase the tension within the band.