The nitrogen-vacancy (NV) colour centre in diamond is a crucial physical system for emerging quantum technologies, including quantum metrology, information processing, and communications, as well as nanotechnologies like biological and subdiffraction limit imaging and tests of entanglement in quantum mechanics. Despite over 50 years of research, the physics of the NV centre remains challenging, with many early assertions now considered false and unresolved issues remaining. This review compiles key empirical and ab initio results from the extensive NV literature into a consistent picture of current understanding. It identifies unresolved issues and examines possible avenues for their resolution. The NV centre, a point defect in diamond with C3v symmetry, consists of a substitutional nitrogen and lattice vacancy pair aligned along the [111] direction. It exists in negative (NV⁻) and neutral (NV⁰) charge states, with distinct optical zero phonon lines (ZPLs) and vibronic bands. The NV⁻ centre has been used in room-temperature demonstrations of quantum registers and spin-photon entanglement, as well as in quantum information processing and nanoscale magnetometry. It has also been proposed for quantum cryptography, single photon generation, and photonic devices. The NV⁻ centre's electronic structure includes a spin triplet ground state (³A₂) and an excited state (³E), with spin multiplicity and fine structure dependent on electric, magnetic, and strain fields. The ground state spin exhibits long coherence times and can be manipulated via microwave and static fields. The NV⁻ centre's optical transitions are highly sensitive to strain and electric fields, enabling tunable optical transitions and spin-flip/conserving transitions. The centre's photostability and ability to generate single photons make it a promising candidate for quantum technologies. The NV⁻ centre's properties, including its optical transitions, spin dynamics, and coherence, have been studied extensively. The ground state spin has the longest room temperature single spin coherence time of any electronic spin in a solid, enabling its coupling with proximal electronic and nuclear spins. The NV⁻ centre's ability to be fabricated in various crystal environments and its robustness to fabrication techniques make it a versatile platform for quantum technologies. The review discusses the electronic structure of the NV⁻ and NV⁰ centres, their charge states, and the challenges in understanding their properties. It highlights the importance of ab initio calculations and experimental studies in resolving these issues. The NV⁻ centre's applications in quantum technologies, including quantum information processing, nanoscale sensing, and metrology, are also discussed. The review concludes with an overview of the current understanding of the NV centre and the ongoing challenges in its study.The nitrogen-vacancy (NV) colour centre in diamond is a crucial physical system for emerging quantum technologies, including quantum metrology, information processing, and communications, as well as nanotechnologies like biological and subdiffraction limit imaging and tests of entanglement in quantum mechanics. Despite over 50 years of research, the physics of the NV centre remains challenging, with many early assertions now considered false and unresolved issues remaining. This review compiles key empirical and ab initio results from the extensive NV literature into a consistent picture of current understanding. It identifies unresolved issues and examines possible avenues for their resolution. The NV centre, a point defect in diamond with C3v symmetry, consists of a substitutional nitrogen and lattice vacancy pair aligned along the [111] direction. It exists in negative (NV⁻) and neutral (NV⁰) charge states, with distinct optical zero phonon lines (ZPLs) and vibronic bands. The NV⁻ centre has been used in room-temperature demonstrations of quantum registers and spin-photon entanglement, as well as in quantum information processing and nanoscale magnetometry. It has also been proposed for quantum cryptography, single photon generation, and photonic devices. The NV⁻ centre's electronic structure includes a spin triplet ground state (³A₂) and an excited state (³E), with spin multiplicity and fine structure dependent on electric, magnetic, and strain fields. The ground state spin exhibits long coherence times and can be manipulated via microwave and static fields. The NV⁻ centre's optical transitions are highly sensitive to strain and electric fields, enabling tunable optical transitions and spin-flip/conserving transitions. The centre's photostability and ability to generate single photons make it a promising candidate for quantum technologies. The NV⁻ centre's properties, including its optical transitions, spin dynamics, and coherence, have been studied extensively. The ground state spin has the longest room temperature single spin coherence time of any electronic spin in a solid, enabling its coupling with proximal electronic and nuclear spins. The NV⁻ centre's ability to be fabricated in various crystal environments and its robustness to fabrication techniques make it a versatile platform for quantum technologies. The review discusses the electronic structure of the NV⁻ and NV⁰ centres, their charge states, and the challenges in understanding their properties. It highlights the importance of ab initio calculations and experimental studies in resolving these issues. The NV⁻ centre's applications in quantum technologies, including quantum information processing, nanoscale sensing, and metrology, are also discussed. The review concludes with an overview of the current understanding of the NV centre and the ongoing challenges in its study.