Modelling and simulation are widely used methods in design and other engineering tasks. Providing libraries of reusable model fragments is a promising way of supporting engineering modelling. The paper discusses means of structuring such libraries in a generic and reusable way. Model content facts can be organized on the basis of the ontologies that are generally important in physical systems engineering. In addition, the paper suggests that such libraries need to support various kinds of modelling assumptions about model fragments. Modelling can be usefully approached as a form of assumption management, as a means of reflecting the metalevel knowledge that is employed by experienced engineers. A categorization of modelling assumptions is proposed that aims at further support for reusability and shareability.