The greatest certified height reached by a commercial airliner as of February 2014 is 60,000 feet. Although greater heights can be dangerous for equipment as well as passengers, there is little turbulence after an aircraft flies above 50,000 feet.
The highest cruising altitude for most airliners is approximately 45,000 feet, depending on the size and construction of the craft. The highest altitude an aircraft can reach is called its "ceiling." If a plane were to attempt to exceed its ceiling, it might suffer engine damage, lose thrust and reach the inability to maintain decompression. Commercial airliners typically reach altitudes of 30,000 feet.