Those who write compact models for use in circuit simulation need a basic understanding of what goes on inside a circuit simulator. This presentation is a short introduction for beginners. It briefly covers how circuit equations (so-called Differential Algebraic Equations, DAEs) are built up and how they are solved to find the circuits’ steady state solutions (by Newton Raphson iteration). It also discusses how circuits are defined (in netlist format), how different kinds of analyses (DC, AC, …