Despite all the scientific efforts to investigate superconductivity, it remains a scientific problem to be solved. Many new superconducting substances are still being discovered. The critical temperature is steadily rising. Physicists have succeeded in explaining many features of the new substances. But nature has more questions than answers, and superconductivity, i.e. high-temperature superconductors, remains a complex and mysterious phenomenon. This book aims to be the simplest of all popular books to explain what superconductivity is, how it was discovered, what its main properties are, where they are applied now and how they are most likely to be widely used in future.