We are deeply indebted to Colin Borys, Jason Surace, Kentaro Nagamine, C. Kevin Xu & Lee Armus who provided original figures from their recent work for this review. The Spitzer IRS ULIRG team who responsible for the spectra shown and described in Section 3 include B. Thomas Soifer, James Houck, Lee Armus, Vassilis Charmandaris, Henrik Spoon, Jason Marshall, Jeronimo Bernard-Salas & Sarah Higdon. CJL and HES are indebted to Yu Gao and the organizers of the Lijiang Workshop on "Extreme Starbursts" which contributed greatly to the focus of this review.