  Multiquark resonances are undoubtedly experimentally observed. The number of
states and the amount of details on their properties has been growing over the
years. It is very recent the discovery of two pentaquarks and the confirmation
of four tetraquarks, two of which had not been observed before. We mainly
review the theoretical understanding of this sector of particle physics
phenomenology and present some considerations attempting a coherent description
of the so called X and Z resonances. The prominent problems plaguing
theoretical models, like the absence of selection rules limiting the number of
states predicted, motivate new directions in model building. Data are reviewed
going through all of the observed resonances with particular attention to their
common features and the purpose of providing a starting point to further
research.
