The possible production of multiquark systems is very important for our understanding of hadrons. A considerable interest in such states started with Jaffe's work in 1977, demonstrating the role of the chromomagnetic interaction in the stability of light multiquarks. Since then, heavy quarks have also been included. A brief survey is presented regarding the evolution of the problem. Some of the ... [Show full abstract] recently observed resonances, named X,Y or Z, are discussed as possible candidates for tetraquarks.