Theory is the essential foundation on which an empirical network study is built. A network theory stipulates a certain, carefully defined network and offers a reason why it relates to other variables. Pinning down what the precise network of theoretical interest is and fleshing out a reason why it matters is what makes up the key preliminary work in empirical networks research design. It can be tempting to rush through this preliminary step, especially when data are readily available. Note that doing so comes with risks. Design blunders are more debilitating in networks research than in other data collection endeavors. Thinking through all aspects of a theoretical setup takes time, but is part of the real work of research design. Taking the time early is an investment in avoiding wasted effort later. This chapter presents a framework to help construct a theory that is maximally useful for guiding empirical research design.