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