Book excerpt: No systematic analysis of optometric clinical data can be undertaken unless the findings are referred to a logical model that allows comparisons to be made of interaction characteristics that exist between the accommodative and convergence mechanisms inherent to the human vision system. Concomitantly, many of the concepts applied during any such analytical process are based on hypothetical constructs. This text re-examines the elements originally proposed by Dr. A.M. Skeffington, and offers a revised insight into how performance changes may occur as individual visual behavior adapts to the impact of environmental demands. In-depth research and extensive references attempt to substantiate the Skeffington paradigm of professional vision analysis. Long-standing challenges to the optometric profession are identified. As presented, the contents are appropriate for use as a classroom text, for reference, and for identifying areas for clinical research.