A Basic Mental Model (BMM) of a mathematical concept is a content-related interpretation that gives meaning to this concept. There exist many suggestions for BMMs in mathematics education, e.g., for natural numbers, functions, derivatives or integrals. In this article we present four BMMs of an equation and discuss the relation between these BMMs and the solving of equations. Particularly, we are interested in the interrelationship between these BMMs and the use of digital technologies in this solving process. These theoretical considerations bring up research questions which should be answered to achieve a better understanding of the concept of equation and the solving processes.