The highest energy electromagnetic waves (or photons) are the gamma rays. Their energies range from millions of electron volts (MeV) to thousands of electron volts (keV), thus their wavelengths are very short (10^-11m to 10^-13m).

Many nuclear reactions and interactions result in the emission of gamma rays. We can take advantage of these interactions for medicinal purposes, especially in cancer treatments, where focused gamma rays can be used to eliminate malignant cells.

Distant galaxies are also prodigious sources of gamma rays, where they are thought to be produced by very hot matter falling onto a massive black hole in the center of the host galaxy. Our atmosphere shields the Earth from most gamma rays produced in deep space.