The autoignition of dimethyl ether (DME), an alternative diesel engine fuel, has been studied at elevated pressures. Ignition delay times were measured in reflected shock experiments at temperatures from 697 to 1239 K and at a nominal pressure of 22-23 bar for DME/air/N-2 mixtures at equivalence ratios of 0.5, 1.0, and 1.5 and with 0-40% N-2 dilution. DME ignition delay times were observed to display three regimes of reactivity (high-temperature, negative-temperature-coefficient (NTC), and low-temperature) characteristic of paraffinic hydrocarbons and were shown to decrease with increasing equivalence ratio and increase with increasing dilution at the conditions studied. Ignition delay time measurements are compared to the detailed kinetic model of Zhao et al. (Zhao et al. Int. J. Chem. Kinet. 2008, 40, 1-18) with remarkable agreement; experiment-model deviations are mostly within the experimental uncertainties. Reaction flux and sensitivity analysis performed with the Zhao et al. model illustrates the importance of H abstraction from DME in controlling high-temperature autoignition (similar to 1200 K). At NTC and low-temperature conditions, the competition between the addition of molecular oxygen to and beta-scission of the methoxymethyl (CH3OCH2) and hydroperoxy-methoxymethyl (CH2OCH2O2H) radicals plays an important role in governing autoignition and its temperature dependence.