A superconductor is a substance that offers zero resistance to the electric current when it drops below a critical temperature. Zero resistance is not the only interesting phenomenon as materials reach their transition temperatures. A second effect is the exclusion of magnetic fields. This is known as the Meissner effect. A light, permanent magnet placed over a superconducting sample will levitate in a stable position above the superconductor. High-speed trains that levitate on strong superconducting magnets have been developed, eliminating the friction normally experienced between the train and the tracks. On April 3, 1997, the Yamanashi Maglev test line in Japan officially opened, and in April 2015, the MLX01 test vehicle attained a speed of 374 mph using strong superconducting magnets.

Superconductors can be categorized into Type I and Type II superconductors.

Thirty pure metals exhibit zero resistivity below their critical temperature and exhibit the Meissner effect, the property of excluding magnetic fields from the interior of the superconductor while the superconductor is at a temperature below the critical temperature. These metals are called Type I superconductors. The superconductivity exists only below their critical temperatures and below a critical magnetic field strength. Type I superconductors have limited practical applications because the strength of the critical magnetic field needed to destroy the superconductivity is quite low.

Type II superconductors have much higher critical magnetic fields and can carry much higher current densities while remaining in the superconducting state. Various ceramics containing barium-copper-oxide have much higher critical temperatures for the transition into a superconducting state. Superconducting materials that belong to this subcategory of Type II superconductors are often categorized as high-temperature superconductors.