Circuit simulation is a standard task for the computer-aided design of electronic circuits. The transient analysis is well understood and realized in powerful simulation packages for conventional circuits. However, further developments in production engineering lead to new classes of circuits, which cause difficulties for the numerical integration. The dimension of circuit models can be quite large (10 5 equations). The complexity of the models demands a higher level of ion. Parasitic effects become dominant. The signal to noise ratio becomes smaller. In this paper, we want to draw attention to three essential problems from a mathematical point of view, the DAE-index, consistent initial values, and asymptotic stability. These topics have been extensively analyzed only recently. We shall illustrate them by some simple examples.