World-systems typically expand. Thus, they incorporate or absorb new peoples, new territories, or both. In situations where more than one world-system exists, sufficient expansion might lead to the merger or combination of one or more into a larger system. Alternatively, a merger might be seen as the incorporation of one world-system by another. The point of this article is to summarize and expand upon discussions of the concepts of incorporation into world-systems and the merger of world-systems. I also provide a brief account of how frontier zones fit into these discussions and conclude with some suggestions of possible directions for future research.