The purpose of the present study was to examine whether the advantages of adopting an external focus would be seen primarily for relatively challenging (postural stability) tasks but not less demanding tasks. To examine this, the authors used balance tasks that imposed increased challenges to maintaining stability. The present results support the view that attentional focus effects emerge only when the support surface sufficiently compromises maintaining a stable posture. In Experiment 1, there was an interaction between task and attentional focus. Specifically, while there were no attentional focus differences for the solid surface task, an external focus resulted in greater stability compared to the control condition for the foam surface task. In Experiment 2, where more challenging balance tasks were used and postural sway was perceptibly increased, external focus benefits emerged relative to both internal focus and control conditions. Thus, it seems the more apparent need to control stability, coupled with the greater coordination demands of Experiment 2, resulted in the external focus benefits seen in previous studies. This confirmed the hypothesis that a certain degree of instability may be a precondition for the attentional focus effects to occur. The present findings suggest that external attentional focus instructions are only beneficial for skills in which errors, or postural instability, tend to be large and performers are more prone to intervene into automatic control processes. External focus instructions seem to inhibit this tendency, whereas internal focus instructions do not. (Contains 2 figures.)