The extensive and scattered literature on vergence eye movements is reviewed. Topics covered include the neural innervation to vergence, active versus passive divergence, muscular control, the accuracy of vergence, the interaction from vergence to accommodation, the independence of vergence and version, the sharing of responsibility with other eye movement controllers, the anatomical and physiological resting point, prediction and learning, drug effects, and the stimuli to vergence, i.e. fusion, accommodation and proximity. Results from ophthalmology, physiology, as well as bioengineering are discussed, using control systems theory as a tool for description and explanation. It is argued that in this area the systems approach is the most appropriate method for interpreting experimental results. Finally, a model that attempts to integrate the results into a theory of vergence is proposed. On the one hand the model suggests possible explanations. On the other, where the model is inadequate it serves to direct attention either to modifications of the theory or to areas where further experimentation is indicated.