In this paper, we obtain several abstract results concerningthe exact controllability of semilinear evolution systems. First, we provethe null local exact controllability of semilinear first-order systems bymeans of the contraction mapping principle; in this case, we do notassume any compactness. Next, we derive the global andor local exactcontrollability of semilinear second-order systems by means of theSchauder fixed-point theorem; in this case, we assume only the embeddingof the related spaces having some compactness, which is reasonablefor many concrete problems. Our main result shows that the observabilityof the dual of the linearized system implies the exact controllabilityof the original semilinear system. Finally, we apply our abstractresults to the exact controllability of the semilinear wave equation.