HFC-134a is being phased out in the European Union in automotive air conditioning systems due to high global warming potential and perceived excessive leakage from systems. HFO-1234yf has been identified as a potential replacement with excellent environmental properties and cooling performance very similar to HFC-134a. The most significant challenge in the transition to this new refrigerant is that it is mildly flammable. Therefore, it was important to understand the impact and associated risk of using a flammable refrigerant in automotive air conditioning systems. First, basic flammability properties of HFO-1234yf were determined. Then leakage and ignition testing were conducted under a range of leak scenarios both in vehicles and during service. Finally, the results generated were incorporated into risk assessments to understand potential risks during automotive vehicle air-conditioning use. For HFO-1234yf, this work was conducted and completed through unprecedented industry cooperation and sharing of information. This paper will highlight the extensive flammability testing and evaluations completed which led to the conclusion that HFO-1234yf is safe for use in automotive air conditioning.