This document discusses theories of complex networks. It introduces scale-free networks and preferential attachment, where new nodes are more likely to connect to existing popular nodes. This leads to a few hubs with many connections and many nodes with few connections, rather than a random distribution. Implications include that popularity begets more popularity, niches are important, older nodes tend to be more connected, and money alone does not guarantee future popularity but connecting to existing popular nodes can help growth. Networks cluster with dense connections within communities and sparser connections between communities.