This "How-To Tips and Tools" article by Sara Lev struck me because it demonstrates how PBL can fit any age, even four- and five-year-olds. Her point was that the "driving question" can come from listening to the students and helping them formulate what they want to do in their terms.

This article succinctly points out why PBL can be so difficult to initiate into a school system. The author notes that our educational systems reward obedience over innovation. Worth thinking about how that can be changed.