  We investigate the unconventional quantum phase transitions in Weyl
semimetals. The emergent boson fields, coupling with the Weyl fermion
bilinears, contain a Wess-Zumino-Witten term or topological $\Theta$ term
inherited from the momentum space monopoles carried by Weyl points. Three types
of unconventional quantum critical points will be studied in order: (1) The
transition between two distinct symmetry breaking phases whose criticality is
beyond Landau's paradigm. (2) The transition between a symmetry breaking state
to a topological ordered state. (3) The transition between $3d$ topological
order phase to trivial disordered phase whose criticality could be traced back
to a $Z_2$ symmetry breaking transition in $4d$. The essence of these
unconventional critical points lies in the fact that the topological defect of
an order parameter carries either a nontrivial quantum number or a topological
term so the condensation of the defects would either break some symmetry or
give rise to a topological order phase with nontrivial braiding statistics.
