Validated pain assessment tools have transformed the ability to identify and quantify pain in livestock, enabling evidence-based analgesic use and welfare monitoring. The Grimace Scale approach - originally developed for mice by Langford et al. - has been adapted for cattle (Bovine Grimace Scale), pigs (Piglet Grimace Scale), sheep (Sheep Pain Facial Action Coding System), and horses (Horse Grimace Scale). These tools assess facial action units including ear position, orbital tightening, cheek tension, and nasal dilation. Clinical validation studies demonstrate high reliability between trained assessors. Composite Measures of Pain (CMP) combine behavioural, postural, and physiological indicators for more comprehensive assessment. Automated pain detection using machine vision and deep learning is under development. Practical clinical application requires brief training; digital apps enable field use. Integration into farm health protocols supports evidence-based treatment decisions and welfare audit documentation.