This paper consists of two parts, the first of which appeared in the previous issue of the journal. It deals with some of the many aspects of modelling physical systems in terms of differential equations (DE), ordinary and partial. Among the issues touched upon are (a) the important role played by the assumptions underlying a DE based mathematical model (Part I), and (b) the interplay between modelling and simulation when using differential equation models (Part II). In a more philosophical vein, there is a section on determinism as well as one on model validation; while fairly general in scope, the concepts examined therein are introduced in direct relation to DE models.