Quick, small and innovative: That's the kind of mission NASA is seeking, and that's what the Mars Scout program is all about. NASA's past missions to Mars–Mariner, Viking, Pathfinder–were mostly large-scale exploratory ventures, with sometimes a decade or more separating launches. Mars Scout missions, scheduled to begin in 2007, can be launched much more frequently and cheaply. Best of all, the program allows small groups of scientists to propose very specific missions on subjects that address cutting-edge questions in their fields. For the first Scout launch, more than 25 proposals were submitted. Two years later, the competition is down to these four. The final decision will be made this summer.