The space of supersymmetric field theories in six dimensions is rich in structure: there exist nontrivial infrared fixed points of the renormalization group, matter contents of chiral theories are strongly constrained by anomaly cancellation and, in particular, there exists an elementary theory which is more fundamental than "gauge theories." The theory is known as the E-string theory. We will see how this theory arises and how one can study its spectrum using nonperturbative methods and dualities.