In this paper, performances of stored-reference (SR), transmitted-reference (TR), and energy detection (ED) based time of arrival (TOA) estimation techniques are analyzed for impulse-radio ultra-wideband (IR-UWB) systems. Various maximum likelihood estimation approaches are investigated under different observation models, and a new estimator that exploits the noise statistics and power delay profile of the channel is proposed. Simulation results show that ED and TR perform well if the sampling rate is much smaller than the Nyquist rate. Also, exploiting the channel and noise statistics considerably improves the accuracy of peak selection.