  To understand large, connected systems, we cannot only zoom into the details.
We also need to see the large-scale features from afar. One way to take a step
back and get the whole picture is to model the systems as a network. However,
many systems are not static, but consisting of contacts that are off and on as
time progresses. This Chapter introduces the mathematical and computational
modeling of such systems and thus an introduction to the rest of the book. We
will cover some of the earlier developments that form the foundation for the
more specialized topics of the other Chapters.
