Not only has the number of launched spacecraft per year exploded recently, but spacecraft are also getting progressively more complex as instruments become more capable and flyby missions give way to remote orbiters, then to rovers and other in situ explorers. To address the software issues in this expanding mission set, JPL started the Mission Data System (MDS) project – an effort to make engineering flight software more straightforward and less prone to error through the explicit modeling of spacecraft state. This paper presents how MDS performs mission planning and execution in the context of managing explicit spacecraft state.