Abstract: The launch of the Fermi Gamma-ray Space Telescope in 2008 has ushered in a new era for the study of the extreme Universe. In addition to ground-breaking improvements in sensitivity and angular resolution, providing the deepest and sharpest view of the high-energy gamma-ray sky, Fermi also has opened a whole new window in the time domain. With a large field of view allowing complete coverage of the sky every 3 hours, Fermi can "catch" rare and exciting transient events in addition to following the behaviour of all gamma-ray emitters as a function of time. In this talk, I will review some of the surprises uncovered by Fermi on timescales from milliseconds to years and describe how these results have advanced our understanding of some of the most extreme phenomena known to astrophysics.