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Many real-world planning problems require generating plans that maximize the parallelism inherent in a problem. There are a number of partial-order planners that generate such plans; however, in most of these planners it is unclear under what conditions the resulting plans will be correct and whether the planner can even nd a plan if one exists. This paper identiies the underlying assumptions about when a partial plan can be executed in parallel, deenes the classes of parallel plans that can be generated by diierent partial-order planners, and describes the changes required to turn ucpop into a parallel execution planner. In addition , we describe how this planner can be applied to the problem of query access planning, where parallel execution produces substantial reductions in overall execution time.