Complex networks are ubiquitous in systems of physical, biological, social or technological origin. Components in those systems range from as large as cities in power grids, to as small as molecules in gene regularity networks. The complexity is embedded in both the heterogeneity across components and the complex (e.g., nonlinear or stochastic) interactions between actors. Pursued collectively in Statistical Physics, Applied Mathematics, Computer Science, and Social Science etc., the interdisciplinary study of such diverse systems using network tools and theories has exploded during the past two decades.