A new dynamic modelling methodology, SLlN, allowing for the analysis of systems defined in qualitative or quantitative terms is presented. Simulation of qualitative characteristics is performed by applying a set of logical rules which include base, tactical, strategic and structural change rules. Quantitative simulation applies traditional system dynamics concepts. To make the transition from qualitative into quantitative modes, logical rules are also used. SLIN is advantageously implemented on a very high level language such as PROLOG, as shown in this paper. To illustrate its potential applications, simple water quality models are included.