Standards and policies intended to safeguard nonhuman animal welfare, whether in zoos, farms, or laboratories, have tended to emphasize features of the physical environment. However, research has now made it clear that very different welfare outcomes are commonly seen in facilities using similar environments or conforming to the same animal welfare requirements. This wide variation is almost certainly due, at least in part, to the important effects of the actions of animal care staff on animal welfare. Drawing mostly on the farm animal literature, we propose that this "human dimension" of animal welfare involves seven components: (1) positive human-animal interaction, (2) consistency and familiarity of keepers, (3) treating animals as individuals and taking account of their personalities, (4) the attitudes and personalities of keepers, (5) the keepers' knowledge and experience, (6) the keepers' own well-being, and (7) the influence of facility design on how keepers and others interact with the animals. We suggest that attention to these human factors provides major scope for improving the welfare of animals in zoos.