After 15 years in construction, the LHC has started its physics program. It is  the most complex scientific instrument ever built, bringing together science and engineering on a grand scale. The 6000 superconducting magnets weighing nearly 50000 tons in total are cooled with 100 tons of superfluid helium. The machine has been built to address some of the most fundamental scientific questions including the origin of mass, dark matter and dark energy and the subtle asymmetry between matter and antimatter responsible for our very existence. Some of these questions will be discussed as well as the engineering principles of this unique instrument.