The complexity of quantum chromodynamics in the non-perturbative domain forces physicist to resort to models in order to describe the phenomena of intermediate energy physics and hence this book develops three different quark models to study light and some heavy mesons: a non relativistic quark model with one-gluon-exchange potential(OGEP) and Instanton-Induced Interaction (III); a semi-relativistic quark model with OGEP and III where the confinement of the quark is modeled with the relativistic harmonic model; a relativistic model where the features of the two previous models are...