This paper proposes the causality relation for mobile multiagent systems and examines whether this relation can be detected in the presence of faulty agents. Towards this end, we first adapt the happened before or the causality relation over events from distributed message-passing systems to mobile multiagent systems. We propose three variants of this new relation -- regular causality, weakly correct causality, and strongly correct causality -- and examine each for its fault-tolerance in the face of Byzantine agents. We establish some fundamental results on the solvability and unsolvability of causality detection in multiagent systems where some of the agents may be Byzantine. Specifically, we show that regular causality and weakly correct causality cannot be detected in the presence of even a single Byzantine process, whereas strongly correct causality, where there is no influence of Byzantine agents, can be detected.