1. Wild animals are used in scientific research in a wide variety of contexts both in situ and ex situ. Guidelines for best practice, where they exist, are not always clearly linked to animal welfare and may instead have their origins in practicality. This is complicated by a lack of clarity about indicators of welfare for wild animals, and to what extent a researcher should intervene in cases of compromised welfare. 2. This Primer highlights and discusses the broad topic of wild animal welfare and the ethics of using wild animals in scientific research, both in the wild and in controlled conditions. Throughout, we discuss issues associated with the capture, handling, housing and experimental approaches for species occupying varied habitat...