Planning and execution systems have been used in a wide variety of systems to create practical and successful automation. They have been used for everything from performing scientific research on the surface of Mars to controlling enemy characters in video games to performing military air campaign planning. After reviewing past work on these various planning and execution systems, we believe that most lack one or more key components contained in another system. To enable future researchers to build more complete systems, and avoid possible serious system failure, we identify the major technical problems any implementer of such a system would have to face. In addition we cite recent solutions to each of these technical problems. We limit our focus to planning and execution for virtual worlds and the unique problems faced therein.