Using carbon dioxide (CO2) for stunning pigs at slaughter is common in Europe. The use of group stunning is amajor advantagewith CO2,which is donewithout restraining the pigs and with minimized human contact. However,high concentrations of CO2 have been known for decades to cause pain, fear and distress in pigs before lossof consciousness, and the stunning method is clearly associated with animal welfare concerns. This studyreviewed the scientific literature to find recent developments or evaluations of alternative methods that couldlead to the replacement of CO2 for stunning pigs at slaughter. Potential alternative methods found in the literaturewere described and then assessed to identify specific research and development needs for their further development.Only 15 empirical studies were found in the search of peer-reviewed literature since 2004, which is lessthan one per year. Furthermore, half of the studies focused on evaluatingmethods to improve high-concentrationCO2 stunning rather than an alternative to CO2. Since no clear alternative has emerged, nor a method to improveCO2 stunning, there is obviously a strong need to focus research and development to find solutions for improvinganimal welfare when stunning pigs at slaughter.