Multi-agent systems result from interactions between individual agents. Through these interactions different kinds of relationships are formed, which can impact substantially on the overall system performance. However, the behaviour of agents cannot always be anticipated, especially when dealing with open and complex systems. Open agent systems must incorporate relationship management mechanisms to constrain agent actions and allow only desirable interactions. In consequence, in this paper we tackle two important issues. Firstly, in addressing management, we identify the range of different control mechanisms that are required and when they should be applied. Secondly, in addressing relationships, we present a model for identifying and characterising relationships in a manner that is application-neutral and amenable to automation.