In order to decipher the underlying rules governing the non-perturbative behavior of quarks and gluons inside the bound states of QCD, we must first determine and understand the spectrum of the theory. In recent years we have seen increased interest in hadron spectroscopy, triggered primarily by the experimental discovery of states that challenged previous existing conventions. Experimental data alone is not sufficient to confirm the existence of a state or provide a detailed picture of its nature. As a result, it is necessary to carry a parallel theoretical program, and lattice QCD is the natural tool to base this effort on. In this talk, I review the progress in determining and understanding the QCD spectrum using lattice QCD.