The Blazar Sequence and Its Physical Understanding

Introduced in 1998 to attempt a first unified view of the broad-band emission properties blazars, blazar sequence has been extensively used past 25 years guide observations as well physical interpretation overall from these galaxies. In this review, we describe evolution along with tremendous advances observational field, particular gamma-ray band. A new version built on TeV-detected objects is also presented. Two extreme classes (MeV and hard-TeV blazars) are included discussion, given their relevance for future observatories. Finally, current understanding at base presented major criticisms sequence.