Electromagnetic Nonreciprocity

N0{0ex}o0{0ex}n0{0ex}r0{0ex}e0{0ex}c0{0ex}i0{0ex}p0{0ex}r0{0ex}o0{0ex}c0{0ex}i0{0ex}t0{0ex}y has emerged as a fundamental concept in modern science and technology. Recent advances in magnet-free nonreciprocal systems have spurred a resurgence of interest in this subject, but have also led to questions and confusion. The authors review nonreciprocity from a global, first-principles perspective, with the aim of establishing a solid foundation for the topic, to resolve previous issues and promote forthcoming research in the field.