America's National Renewable Energy Laboratory (NREL) has simplified life for wind turbine manufacturers by confirming that its design codes are now approved for worldwide turbine certification. Although the design codes have been used in the United States, until recently they were not accepted by certifying agencies in Europe. NREL and Germanischer Lloyd (GL) of Hamburg, Germany, worked together to approve the codes for calculating onshore wind turbine loads. Previously, manufacturers with wind turbines approved under the NREL code have had to re-evaluate designs using European codes or hire European consultants, a process that increased the time and cost of certifying wind turbines. The two NREL codes are known as FAST (Fatigue, Aerodynamics, Structures, and Turbulence) and ADAMS (Automatic Dynamic Analysis of Mechanical Systems). FAST is a comprehensive aeroelastic simulator capable of predicting both the extreme and fatigue loads of horizontal axis wind turbines with two and three blades. ADAMS is a general purpose, multibody dynamics code with unlimited degrees of freedom that is also used to model robots, satellites and cars. FAST is limited to standard types of horizontal-axis wind turbines. ADAMS can model a wider range of wind turbines. To gain acceptance of the codes, NREL and GL ran a comparison between FAST, ADAMS, GL's DHAT, and Garrad Hassan's GH Bladed, a widely used European code. After the tests, GL granted NREL a certificate for FAST and ADAMS for the calculation of onshore wind turbine loads for design and certification.