A new low global warming refrigerant, CF(3)CF=CH(2) (HFO-1234yf), has been developed to replace HFC-134a in automotive air conditioning systems. HFC-134a is being phased out in the European Union due to high global warming potential. HFO-1234yf is highly energy efficient, exhibits low toxicity, and can potentially be used in systems currently designed for refrigerant HFC-134a with minimal modifications. Significant work has been completed to assess the flammability characteristics of HFO-1234yf. Though it is flammable per ASTM E-681, it is significantly less so than HFC-152a which has also been considered as an HFC-134a alternative. HFO-1234yf has a relatively high lower flammability limit at 6.2 vol% in air Computational Fluid Dynamics (CFD) modeling has shown it is very unlikely the lower flammability limit will be reached when the refrigerant is leaked into the passenger compartment of a vehicle. HFO-1234yf also has a high minimum ignition energy (MIE) from 5,000 to 10,000 mJ which indicates there may be very lbw potential ignition sources in a vehicle. It also has very low burning velocity at 1.5 cm/sec indicating low potential for damage should an ignition occur. Several automotive industry groups have completed extensive risk assessments which have concluded HFO-1234yf is safe to use as a refrigerant in vehicles. This article will review flammability test results and challenges that were overcome to measure flammability of a mildly flammable refrigerant as well as CFD modeling work for vehicle leakage scenarios. (C) 2009 American Institute of Chemical Engineers Process Saf Prog 29: 150-154, 2010