Abstract This paper describes SEMoLa (simple, easy, modelling language), a declarative modelling language developed at the Department of Agricultural and Environmental Sciences of the University of Udine (Italy). SEMoLa language is flexible and particularly suited to model complex ecological and environmental systems and to manage different types of information. Based on system dynamics principles and using an integrated approach, SEMoLa simplifies the routinely tasks of implementing, debugging and evaluating computer simulation models. This modelling language is implemented in a framework which allows to easily analyse and represent simple and complex system, permitting simulation model development, even to non-mathematically and programming skilled users. This paper presents SEMoLa system ontology and its language, together with example models of different type of systems: state-based and element-based, continuous and event-driven, deterministic and stochastic.