In the absence of an adequate visual stimulus, the eyes are typically converged by approximately 0.25 to 0.75 meter angles (MA). This vergence response (VR) was believed to reflect the level of tonic innervation to the extraocular muscles, and accordingly has been termed tonic vergence (TV). However, this estimation fails to consider the magnitude of the anatomical position of rest. The true typical value of TV is approximately 23 degrees. This paper will consider various aspects of this parameter, including both clinical and laboratory methods of measurement, and the relationship between TV and the distance heterophoria. In addition, the role of vergence (or prism) adaptation, i.e., the apparent change in TV after periods of sustained fixation, is discussed. This shift appears to result from the relatively prolonged decay of the slow fusional vergence response (VR), with no evidence for a change in the level of tonic innervation. On occasion, the decay of slow fusional vergence may take hours or even days to reach completion. This extended rate of decay will have a significant impact upon the clinical measurement of a number of binocular parameters, most notably the assessment of heterophoria under truly dissociated conditions (i.e., in the absence of any fusional VR). Furthermore, both the magnitude and rate of decay of vergence adaptation appear to vary with age, as well as the presence of oculomotor imbalance. It is concluded that the output of the slow fusional vergence mechanism, as reflected by the degree of vergence adaptation, makes a major contribution to the aggregate, sustained VR in most visually normal patients.