Since the 1970s, research about zoo visitors' effects on the welfare of nonhuman animals in captivity has intensified. Numerous studies have shown that characteristics such as visitor presence, density, activity, size, and position are associated with animal behavioral and—to alesser extent physiological—changes. Studies usually interpret these changes as negative (undesirable) or positive (enriching), but it remains unclear whether they significantly impinge on animal welfare. To make confident conclusions about visitors' effects necessitates more studies using (a) a wider range of animal groupings, (b) measures of stress, (c) visitor-animal variables, and (d) other methodological improvements In the meantime, in addition to further research, individual zoos need to emphasize (a) monitoring the stress indicators of their captive animals, (b) observing visitor behavior, and (c) ensuring that staffs are aware of the “visitor effect” concept.