This paper addresses the problem of obtaining an efficient estimator for sensor node self-localization using time of arrival (TOA) measurements. A sequential scheme is proposed that consists of closed-form single node localization and sensor position refinement algorithms as primitive. The sequential scheme is able to locate sensor nodes with accuracy approaching the CRLB and it can be realized in both centralized and distributed manners. To combat the communication load for a large-scale network, a reduced-complexity version is also proposed with only slight degradation in the estimation accuracy.