A comprehensive summary of current research into multiquark hadrons, describing them in terms of constituent quarks, gluons and compact diquarks.

Ahmed Ali is an Emeritus staff member in Theoretical Physics at the high-energy physics laboratory, Deutsches Elektronen-Sychrotron (DESY), Hamburg and an Emeritus Professor of Physics at the University of Hamburg. Working on the phenomenology of high-energy physics, his main research interests are flavor physics, QCD, and multiquark hadrons. He has worked as a scientific associate at CERN for several years and is also a fellow of the American Physical Society.

Preface; 1. Introduction; 2. XYZ and Pc phenomenology; 3. Color forces and constituent quark model; 4. Hadron molecules; 5. Light scalar mesons; 6. Mass formulae for P-wave, qq mesons; 7. Compact tetraquarks; 8. The Xu Xd puzzle; 9. Y states as P-wave tetraquarks; 10. Pentaquark models; 11. Tetraquarks in large N QCD; 12. QCD sum rules and lattice QCD; 13. Phenomenology of beauty quark exotics; 14. Hidden heavy avour tetraquarks ¿ overview; 15. Tetraquarks with double heavy quarks; 16. Outlook; Appendix A. Low energy p ¿ n scattering amplitude; Appendix B. Wigner's 6-j symbols; References; Index.