HOST: A General Helicopter Simulation Tool for Germany and France. The paper presents the main features of the HOST (Helicopter Overall Simulation Tool) software. It shows how the open architecture allows easy evolutions of the physical models and any association of them in various application fields. The main functions of HOST like trim calculations, time domain simulation and equivalent linear system determination are briefly described. Some examples of application are given for handling qualities, rotor stability in forward flight, loads calculations and inverse simulation. This new helicopter simulation code was chosen as a common basis for tool harmonisation between Eurocopter, ONERA and DLR. Some early achievements of this cooperation concerning dynamic engine models, parametric wake distortion, induced velocity models and parametric identification are presented.