String theory was originally invented to explain the spectrum and scat-tering of mesons. Later on, partially due to the failure to explain the deep inelastic experiment, string theory has been abandoned as the description of hadrons and Quantum Chromo Dynamics (QCD) took over. The focus of string theory since then has moved to superstring theories accommodating quantum gravity and potentially the unification of all matter and interac-tions.

A landmark in the interplay between string and quantum field theories has been the AdS/CFT holographic duality of J. Maldacena. The latter is an equivalence between 10d superstring theory on AdS₅×S⁵ space-time and a 4d N = 4 Super Yang Mills gauge theory. I will very briefly present this duality. I will then show how to invoke a less symmetric string background which corresponds to non-supersymmetric confining theory and to incorporate also quarks into the equivalence via flavor branes. In such setups a meson is described as a rotating string in 5d curved space-time. I will show how to map these strings into rotating strings in flat space-time with massive endpoints. These masses are neither the QCD masses nor the constituent quark masses but rather a new type of “ string endpoint mass”. We determine the classical spectrum of such strings and discuss its quantization. We present a detailed fit of this model to the experimental data of mesons from light ones and all the way to “beautiful ones”. We extract the best values of the masses, string tension and intercept.

In the context of holography baryons are described as Dp branes wrap-ping a p-cycle and connected to flavor branes by N_c strings. Such string configurations are mapped for N_c = 3 into a Y shape or a straight string connecting a quark and a di-quark. I will show that the comparison with data favors the latter option and augment it with a theoretical explanation. Again there is a nice fit between the experimental data and the model which is in accordance with the parameters extracted from the mesonic fit.

The third type of a hadron is the glueball which is described in terms of a closed holographic string. Glueball has not been isolated and identified so far. I will propose a method to do it based on the fact that the string tension of the glueball should be twice that of the meson.

The decay width of holographic stringy mesons is computed and is shown to yield a result similar to the one used in phenomenological models like the Lund model.

I will end with a list of open questions.