This new book was written to put a scientific, mathematical and economic foundation under the young subject of financial systemic risk based on concepts of network science. Consequently, the book places certain modelling concepts as core to the subject. Random financial networks (RFNs) define the system within which financial contagion propagates. Seed events are significant market events, either endogenous or exogenous, that are sufficient to put the financial system into the early stages of a contagion or crisis period. Cascade mechanisms (CMs) such as asset fire sales or default resolution, are the idealized behavioural responses banks are assumed to make when they become stressed and that are responsible for creating stress in other banks, thereby propagating financial contagion. Cascade equilibriums are the final state of a modelled financial crisis, where we can assess the total damage caused by any number of systemic risk measures. By focussing on these concepts, we try to identify the most salient features that are likely to drive future crises.