A simulation environment for the dynamic simulation of paper or board mills has been developed. APMS (advanced paper mill simulator) provides tools for fast and easy modelling of the plant comprising e.g. stock preparation, approach system, wet end and dry end of the machine. The models are based on the first principles of physics and chemistry whenever it is possible. This mechanistic modelling principle with efficient equation solution techniques ensures fast and accurate simulation for the purposes of process and automation design, analysis and operator training.