Federal government agencies and an industrial industry consortium have funded Carnegie Mellon University (CMU) for the development of Explorer™, a long range, un-tethered, modular inspection robot for the visual inspection of 6" and 8" natural gas distribution system pipelines. The robot can be launched into the pipeline under live conditions utilizing a commercial no-blow system via a specially designed attachment, and can negotiate diameter changes, 45-deg and 90-deg bends and tees, as well as inclined and vertical pieces of the piping network. The modular design of the system allows it to be expanded in the near future to include additional inspection and/or repair tools. The range of the robot is an order of magnitude higher than present state-of-the-art inspection systems and is expected to fundamentally alter the way gas utilities maintain and manage their systems. A prototype system has been built, and has undergone extensive laboratory system testing and field demonstrations in 2004 and 2005. This paper will describe the overall engineering design and functionality of the Explorer™ system, as well as the results of the field-trial data to date.