Entity-Relationship (ER) modelling is a popular technique for data modelling. Despite its popularity and widespread use, it lacks a firm semantic foundation. We propose a translation of an ER-model into relation algebra, suggesting that this kind of algebra does provide suitable mechanisms for establishing a formal semantics of ER modelling. The work reported on here deals first with the techniques necessary for the translation, thus constructing a static view of an ER-model in an abstract setting of what might be called logic without variables. We then undertake a detailed analysis of the insertion and deletion operations for an ER-model represented in terms of the relation calculus.