Engineering has long gravitated toward great human ambitions: navigation of the oceans, travel to the moon and back, Earth exploration, national security, industrial and agricultural revolutions, communications, and transportation. Some ambitions have been realized, some remain unfulfilled, and some are yet to be determined. In 2008 a committee of distinguished engineers, scientists, entrepreneurs, and visionaries set out to identify the most important, tractable engineering system challenges that must be met in this century for human life as we know it to continue on this planet. For the forum at the National Academy of Engineering's 2015 annual meeting, 7 of the 18 committee members who formulated the Grand Challenges for Engineering in 2008 reflected on what has happened in the seven year since. Grand Challenges for Engineering: Imperatives, Prospects, and Priorities summarizes the discussions and presentations from this forum.