The aim of this chapter is to build a behavioral system model of proactivity based on attachment theory and elaborate the role of attachment security in shaping the operation of the behavioral system. The chapter firstly indicates that proactive behavior, as a form to challenge the status quo, can be conceptualized as a form of exploration in which instigators aim to master their environment through self-directed change efforts. Next, building on the idea of exploration behavioral system from attachment theory, the chapter offers a behavioral system of proactivity specifically. As a behavioral system is operated in a goal-corrected manner, in which an individual regulates her/his behavior and goals in a feedback loop, the behavioral system of proactivity incorporates the process perspective of proactivity and the three identified motivational mechanisms of employee proactivity reviewed in Chapter 1. Finally, the chapter elaborates how attachment security can influence the operation of the behavioral system of proactivity, building a relational foundation for proactivity.