Event visualization tools are commonlyused to facilitate the debugging of distributed applications. Although these tools provide a graphical view of distributed executions, they are frequently insucient for full debugging purposes. The need for traditional debugging operations isoftenoverlookedwhen buildingthese tools. One of the most useful operations in traditional debuggers is single stepping. However, the diculties faced when dealing with partially ordered-executions have deterred the development of a single-stepping method for concurrent processes. This paper describes a single-stepping operations suitable for eventvisualization tools. Three dierent methods for single stepping are presented: global-step, r and step-in. Abstraction techniques are commonly used to reduce the overwhelming amount of detail presented to the user. Accordingly, single stepping in the presence of abstraction is considered. The operations dened in this paper have been implemented in Poet, a Partial Order...