Ammonia (NH3 or R717) is an important refrigerant whose flow boiling heat transfer needs to be determined in many engineering applications. There have been some studies evaluating the correlations of flow boiling heat transfer coefficients for NH3. However, the number of the correlations evaluated or the number of data points used was limited, which resulted in inconsistent results. This work presents a comprehensive study of the applicability of existing correlations of flow boiling heat transfer coefficients to NH3. From seven independent laboratories, a database consisting of 1157 experimental data points of NH3 flow boiling heat transfer is compiled. The experimental parameter ranges cover mass flux from 10 to 600kg/m(2)s, heat flux from 2.0 to 240 kW/m(2), vapor quality from 0.002 to 0.997, saturation pressure from 0.19 to 1.6 MPa, and channel inner diameter from 1.224 to 32mm. Based on the NH3 database, 37 correlations are evaluated and analyzed. The results show that the best correlation has a mean absolute deviation of 40.9%, indicating the need for developing a more accurate correlation for NH3 flow boiling heat transfer. Several topics worthy of further studies are identified.