Magnetic random access memory (MRAM) is a new class of solid state memory which is based on the spin of the electron (as opposed to its charge). This technology is a relative newcomer to the constellation of non-volatile memories, and offers a unique combination of non-volatility, density and speed. I will review the scientific discoveries that enabled the first MRAM chips, and then will discuss several distinct generations of the technology which have evolved in the last decade, including both field-switched and spin-torque-switched devices. Finally I will describe promising new structures which are currently under development which exploit the separation of spin and charge to allow more reliable and lower power operation.