Abstract-We consider the problem of maximizing the lifetime of coverage (MLCP) of targets in a wireless sensor network with battery-limited sensors. We first show that the MLCP cannot be approximated within a factor less than ln n by any polynomial time algorithm, where n is the number of targets. This provides closure to the long-standing open problem of showing optimality of previously known ln n approximation algorithms. We also derive a new ln n approximation to the MLCP by showing a ln n approximation to the maximum disjoint set cover problem (DSCP), which has many advantages over previous MLCP algorithms, including an easy extension to the k-coverage problem. We then present an improvement (in certain cases) to the ln n algorithm in terms of a newly defined quantity "expansiveness" of the network. For the special one-dimensional case, where each sensor can monitor a contiguous region of possibly different lengths, we show that the MLCP solution is equal to the DSCP solution, and can be found in polynomial time. Finally, for the special two-dimensional case, where each sensor can monitor a circular area with a given radius around itself, we combine existing results to derive a 1+ǫ approximation algorithm for solving MLCP for any ǫ > 0.