2 1 Introduction
number of key areas. For example, in mathematics the formal underpinnings were
systematically laid, with K
¨
onig [92] cited as the first key architect. The theory of
electrical circuits has always had a substantial network component, going back to
work of Kirchoff, and similarly the study of molecular structure in chemistry, going
back to Cayley. As the fields of operations research and computer science grew
during the mid-1900s, networks were incorporated in a major fashion in problems
involving transportation, allocation, and the like. And similarly during that time
period, a small subset of sociologists, taking a particularly quantitative view towards
the topic of social structure, began developing the use of networks in characterizing
interactions within social groups.
More recently—starting, perhaps, in the early to mid-1990s—there has been an
explosion of interest in networks and network-based approaches to modeling and
analysis of complex systems. Much of the impetus for this growth derives from
work by researchers in two particular areas of science: statistical physics and com-
puter science. To the former can be attributed a seminal role in encouraging what
has now become a pervasive emphasis across the sciences on understanding how
the interacting behaviors of constituent parts of a whole system lead to collective
behavior and systems-level properties or outcomes. Indeed the term complex system
was coined by statistical physicists, and a network-based perspective has become
central to the analysis of complex systems. To the latter can be attributed much of
the theory and methodology for conceptualizing, storing, manipulating, and doing
computations with networks and related data, particularly in ways that enable effi-
cient handling of the often massive quantities of such data. Moreover, information
networks (e.g, the World Wide Web) and related social media applications (e.g.,
Twitter), the development of which computer scientists have played a key role, are
examples of some of the most studied of complex systems (arguably reflecting our
continued fascination with studying ourselves!).
More broadly, a network-based perspective recently has been found to be useful
in the study of complex systems across a diverse range of application areas. These
areas include computational biology (e.g., studying systems of interacting genes,
proteins, chemical compounds, or organisms), engineering (e.g., establishing how
best to design and deploy a network of sensing devices), finance (e.g., studying the
interplay among, say, the world’s banks as part of the global economy), marketing
(e.g., assessing the extent to which product adoption can be induced as a type of
‘contagion’), neuroscience (e.g., exploring patterns of voltage dynamics in the brain
associated with epileptic seizures), political science (e.g., studying how voting pref-
erences in a group evolve in the face of various internal and external forces), and
public health (e.g., studying the spread of infectious disease in a population, and
how best to control that spread).
In general, two important contributing factors to the phenomenal growth of
interest in networks are (i) an increasing tendency towards a systems-level perspec-
tive in the sciences, away from the reductionism that characterized much of the
previous century, and (ii) an accompanying facility for high-throughput data collec-
tion, storage, and management. The quintessential example is perhaps that of the
changes in biology over the past 10–20years, during which the complete mapping