NV magnetometry using nitrogen-vacancy (NV) defects in diamond offers high sensitivity and nanoscale resolution for detecting weak magnetic fields. The NV defect, a substitutional nitrogen atom combined with a vacancy in diamond, has a spin triplet ground state that can be manipulated and read out optically. This allows for high magnetic field sensitivity and spatial resolution, making it ideal for applications in nanoscale imaging, mesoscopic physics, and biology. The NV defect's long spin coherence time and ability to detect magnetic fields through optical means make it a promising tool for quantum sensing. The sensitivity of NV magnetometers is influenced by factors such as spin coherence time, detection efficiency, and the use of ensembles of NV defects. Techniques like dynamical decoupling and side-collection geometries improve detection efficiency and reduce noise. Ensembles of NV defects enhance sensitivity by increasing the number of spins, but may reduce spin readout contrast. Infrared absorption-based magnetometry offers high sensitivity by utilizing spin-dependent infrared transitions. Overall, NV magnetometry has shown great potential for high-precision magnetic field detection and imaging at the nanoscale.NV magnetometry using nitrogen-vacancy (NV) defects in diamond offers high sensitivity and nanoscale resolution for detecting weak magnetic fields. The NV defect, a substitutional nitrogen atom combined with a vacancy in diamond, has a spin triplet ground state that can be manipulated and read out optically. This allows for high magnetic field sensitivity and spatial resolution, making it ideal for applications in nanoscale imaging, mesoscopic physics, and biology. The NV defect's long spin coherence time and ability to detect magnetic fields through optical means make it a promising tool for quantum sensing. The sensitivity of NV magnetometers is influenced by factors such as spin coherence time, detection efficiency, and the use of ensembles of NV defects. Techniques like dynamical decoupling and side-collection geometries improve detection efficiency and reduce noise. Ensembles of NV defects enhance sensitivity by increasing the number of spins, but may reduce spin readout contrast. Infrared absorption-based magnetometry offers high sensitivity by utilizing spin-dependent infrared transitions. Overall, NV magnetometry has shown great potential for high-precision magnetic field detection and imaging at the nanoscale.