  The exploration of energies above the open-flavor threshold in the meson
spectra has led to the appearance of unexpected states difficult to accommodate
in the naive picture of a bound state of a quark and an antiquark. Many of such
states are located close to meson-meson thresholds, which suggests that
molecular structures may be a relevant component in the total wave function of
such resonances.
  In this work, the state of meson-meson molecules calculations is reviewed,
using a non-relativistic constituent quark model that has been applied to a
wide range of hadronic observables, and therefore all model parameters are
completely constrained. The model has been able to reproduce, among others, the
properties of the X(3872), described as a mixture of $c\bar c$ and $D\bar
D^\ast$ states, or the spectrum of the P-wave charm-strange mesons, which are
well reproduced only if $DK$ and $D^\ast K$ structures are taken into account.
  We show that such constituent quark model, which is able to describe the
ordinary heavy meson spectra, is also capable of providing a good description
of many new states recently reported.
