The remarkably strong radar echoes from the summer polar mesosphere have been an enigma to atmospheric and radar scientists since their discovery more than a decade ago. Since then, more sophisticated radar experiments and in situ rocket measurements have shed some light on the underlying physics and chemistry, and theories have been formulated to explain the generation of the intense radar backscatter and the remarkable physical conditions associated with it. First, we review the key observations and examine the proposed theories. We then evaluate the progress that has been made in understanding this phenomenon and explore its connection to global change, to the newly recognized material referred to as a dusty plasma, and to the highest clouds in the Earth's atmosphere. Finally, we end with suggestions for future research.