In this article I discuss the concept of two-mode networks and outline future directions for research. Most social networks consist of only one type of node, or type of observational unit. For example, students may be friends with other students of the same school, or employees may work with other employees in the same corporation. These kinds of social networks are called one-mode networks. A characteristic of one-mode networks is that every node can, in principle, be connected to every other node. For instance, each student can be friends with all the other students in the same school, or each employee works with all other employees in the same corporation. The reference to the word mode comes from the language of matrix algebra, in which a mode refers to a distinct set of entities in a matrix. Many social networks, however, are two-mode networks, which entail additional complexities and issues, as outlined in this article.