Synchrony continues to be an important concern in concurrent programming. Existing languages and models have introduced a great diversity of constructs for expressing and managing synchronization among sequential processes or atomic actions. This paper puts forth a model in which synchrony is viewed as a relation among atomic actions, a relation which may evolve with time. The model is shown to be convenient for expressing formally the semantics of synchrony as it appears in many of the languages and models proposed to date. Among such models Swarm is singled out for its use of dynamic synchrony. The Swarm notation is briefly reviewed. A new concurrent algorithm for the Leader Election problem provides a vehicle for illustrating the use of dynamic synchrony in Swarm.