  The last ten years have opened up a new parameter space in time-domain
astronomy with the discovery of transients defying our understanding of how
stars explode. These extremes of the transient paradigm represent the brightest
- called superluminous supernova - and the fastest - known as fast, blue
optical transients - of the transient zoo. The number of their discoveries and
information gained per event have witnessed an exponential growth that has
benefited observational and theoretical studies. The collected dataset and the
understanding of such events have surpassed any initial expectation and opened
up a future exploding with potential, spanning from novel tools of
high-redshift cosmological investigation to new insights into the final stages
of massive stars. Here, the observational properties of extreme supernovae are
reviewed and put in the context of their physics, possible progenitor scenarios
and explosion mechanisms.
