Building on existing literature, we conceptualize a firm's business model as a system of interdependent activities that transcends the focal firm and spans its boundaries. The activity system enables the firm, in concert with its partners, to create value and also to appropriate a share of that value. Anchored on theoretical and empirical research, we suggest two sets of parameters that activity systems designers need to consider: design elements - content, structure and governance - that describe the architecture of an activity system; and design themes - novelty, lock-in, complementarities and efficiency - that describe the sources of the activity system's value creation.