Natural gas transmission pipelines have proven to be a safe and efficient means for transporting the trillions of cubic feet of natural gas used annually in the United States. Since the peak of construction of these pipelines occurred between 1950 and the mid-1960s, their average age is now over thirty years. However, replacement of these pipelines because of age would be prohibitively expensive and unnecessary. Preventive maintenance and rehabilitation programs put into practice by the pipeline industry provides the key to ensuring the continued integrity of the transmission pipeline system. This article reviews the preventive maintenance practices commonly used by the gas industry. These practices include right-of-way patrols, corrosion control procedures, in-line inspection with intelligent or smart pigs that inspect the pipe while traveling through the inside of the pipe, direct access inspection of the pipe from bellhole excavations, and hydrostatic retesting of pipelines. When pipelines are properly maintained, these practices can ensure the integrity and long-term serviceability of transmission pipelines well into the 21st Century.