  The past seventeen years have witnessed tremendous progress on the
experimental and theoretical explorations of the multiquark states. The
hidden-charm and hidden-bottom multiquark systems were reviewed extensively in
[Phys. Rept. 639 (2016) 1-121]. In this article, we shall update the
experimental and theoretical efforts on the hidden heavy flavor multiquark
systems in the past three years. Especially the LHCb collaboration not only
confirmed the existence of the hidden-charm pentaquarks but also provided
strong evidence of the molecular picture. Besides the well-known $XYZ$ and
$P_c$ states, we shall discuss more interesting tetraquark and pentaquark
systems either with one, two, three or even four heavy quarks. Some very
intriguing states include the fully heavy exotic tetraquark states $QQ\bar
Q\bar Q$ and doubly heavy tetraquark states $QQ\bar q \bar q$, where $Q$ is a
heavy quark. The $QQ\bar Q\bar Q$ states may be produced at LHC while the
$QQ\bar q \bar q$ system may be searched for at BelleII and LHCb. Moreover, we
shall pay special attention to various theoretical schemes. We shall emphasize
the model-independent predictions of various models which are truly/closely
related to Quantum Chromodynamics (QCD). There have also accumulated many
lattice QCD simulations through multiple channel scattering on the lattice in
recent years, which provide deep insights into the underlying
structure/dynamics of the $XYZ$ states. In terms of the recent $P_c$ states,
the lattice simulations of the charmed baryon and anti-charmed meson scattering
are badly needed. We shall also discuss some important states which may be
searched for at BESIII, BelleII and LHCb in the coming years.
