A center is a function that associates with every finite connected and undirected graph a nonempty subset of its vertices. These functions play an important role in networks such as social or interorganizational networks. Centers capture notions like: being a focal point of communication, being strategically located, ability and willingness to participate in strategic alliances, and the like. We focus on the conceptual issue of what makes a position in a graph a central one and investigate some possible concepts of centrality in relation to various properties. Characterizations of the uncovered center, the median, and degree center are presented, where each of these centers is defined for arbitrary connected undirected simple, and possibly cyclic, graphs.