**5. Conclusions**

While there are some identified limitations, grimace scales appear to be a valid tool for pain assessment in many mammalian animals, and have many benefits compared to non-grimace pain assessment techniques. Due to the simplicity of spontaneous use, non-invasive application, repeatability of results, interobserver and intraobserver reliability and ease of training, the use of grimace scales should be more frequently considered as an important parameter of interest in research and animal wellbeing. In addition, this technique has the capacity to satisfy the requirement for refinement in accordance with the 3Rs. Additional research into the use of grimace scales is required for other species, pain-related or other specific studies, and further validation.

**Author Contributions:** Conceptualization, S.C. and T.B.; writing—original draft preparation, S.C.; writing—review and editing, S.C. and T.B.; visualization, S.C.; supervision, T.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** Special thank you to Manuel Christie, Natalie Roadknight, Kat Littlewood, and Natarsha Williams for their assistance in the preparation of this manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.
