Large variation exists amongst patients with regards to treatment outcomes following functional appliance treatment in growing children. Various factors have been assessed with regard to this variation, but evidence is scarce. Recent studies suggest that the initial condition of the masticatory muscles may be one of the factors that influences treatment and post-treatment functional appliance outcomes. Children with weaker masticatory muscles show greater dentoalveolar change, as witnessed by incisor compensation and molar movement. Following functional appliance treatment, children who show greater dentoalveolar treatment change may also be those with a more likely post-treatment sagittal relapse. The gonial angle may also be a variable determining treatment outcomes with functional appliances in that more incisor compensation and a greater likelihood for relapse is evident in those with a more open gonial angle. The gonial process is the site of muscle attachment for the masseter and median pterygoid muscles, and the thickness and force of these muscles can have an effect on the process and its contribution to mandibular morphology. By extrapolation, cephalometric analysis of the gonial angle can perhaps provide insight into the amount of incisor proclination expected to be observed during and after functional appliance treatment.