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 find a plan if one exists. This paper identifies the underlying assumptions about when a partial plan can be executed in parallel, defines the classes of parallel plans that can be generated by different partial-order planners, and describe 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. Parallel execution plans, Partial-order planning, Resources, Execution