This part treats the co-evolution of networks and behavior. The word “behavior” here includes behavior proper but also actors’ cognitions, motivations, attitudes, performance, etc. These co-evolution models make it possible to operationalize theories and test hypotheses about the conditions under which actors’ individual variables are changing in response to their network environment, while acknowledging that simultaneously, this network environment may change in response to the actors’ individual variables. Chapter 11 discusses the mutual patterning of the way in which a behavior can depend in its changes on a network and the way in which a network can depend in its changes on a behavior. Descriptive measures for these mutual dependencies are given in Chapter 12. Chapter 13 presents the elaboration of the stochastic actor-oriented model for network–behavior co-evolution, where “co-evolution” refers to the interdependent dynamics of two or more dependent variables. Effects representing social influence are presented in Chapter 14. The more specific case of innovation diffusion is treated in Chapter 16. Chapter 15 treats methods for parameter estimation and gives advice for model specification. The second part finishes with Chapter 17 on goodness of fit and other uses of simulated data.