We describe a method, called the iterative method, which can be used to construct equilibrium N-body models with a given mass distribution and with given kinematic properties, parameters, or constraints. This method is both very simple and very powerful and can be used for mass distributions with an arbitrary geometry and a large variety of kinematical constraints. Applications of this method include the creation of initial conditions for N-body simulations and the modelling of galaxies from their photometric and kinematic observations.