Superconductivity is one of the most fascinating scientific discoveries of the twentieth century, the mysteries of which have pervaded into the current century and captures the attention of many scientists to date. Superconductivity was discovered in 1911 by H. Kamerlingh Onnes in Leiden [13], after he had perfected liquefaction of helium a few years before, allowing him to reach temperatures as low as 4.2 K. Onnes won the Nobel Prize in Physics in 1913 for this discovery of new state of matter. Elemental mercury was the first discovered superconductor, through its hallmark defining property of a zero-resistance electrical conductivity state (also known as perfect conductivity) upon cooling down to liquid helium temperatures. This chapter presents a historical overview of superconductivity and the basic theoretical foundations of superconductivity such as the electrodynamics of superconductors, the Ginzburg-Landau theory and the Bardeen-Cooper-Schrieffer theory that describe most observed properties of conventional elemental superconductors. The properties of Josephson junctions, which bring out the macroscopic quantum nature of the superconducting wave function and central to the experimental results described in other chapters of this book are described in detail. In addition, the symmetry requirements for non-reciprocal transport in superconductors and the current state of the art in supercurrent diodes are introduced to the reader.