In many practical applications of wireless sensor networks, it is crucial to accomplish the localization of sensors within a given time bound. We find that the traditional definition of relative localization is inappropriate for evaluating its actual overhead. To address this problem, we define a novel problem called essential localization, and present the first rigorous study on the essential localizability of a wireless sensor network within a given time bound. We propose an efficient distributed algorithm for timebounded essential localization over a sensor network, and evaluate the performance of our algorithm with extensive simulations.