This review article discusses the mechanisms of electron emission and electrical breakdown in nanogaps, focusing on the transition between different emission mechanisms, the role of surface effects, and the impact of high electric fields and temperatures. It summarizes recent theories, experiments, and atomistic simulations related to these phenomena. The process of electrical breakdown in nanogaps involves nano-protrusion growth, electron emission, thermal runaway, and plasma formation. The review emphasizes the importance of understanding these processes for the design and operation of nanoscale electronic devices. Electron emission in nanogaps can occur through several mechanisms, including direct tunneling, field emission, and space-charge limited emission. The Fowler-Nordheim (FN) equation describes field emission at low current densities, while the Child-Langmuir (CL) law describes space-charge limited emission at high current densities. Recent studies have shown that quantum effects become significant at nanoscales, and the CL law can be modified to account for these effects. The review also discusses the influence of electrode geometry, surface roughness, and material properties on electron emission and breakdown. Experiments on electron emission and breakdown in nanogaps have been conducted using various techniques, including fixed and adjustable nanogap configurations. These experiments have revealed the effects of space-charge, electrode deformation, and other factors on the breakdown process. The review highlights the importance of understanding these factors for the development of reliable nanoscale electronic devices. The review also discusses the role of atomistic simulations in understanding the behavior of nanogaps. These simulations have provided insights into the growth of nano-protrusions, the thermal runaway of nano-protrusions, and the formation of plasma in nanogaps. The review concludes that further research is needed to fully understand the complex mechanisms of electron emission and electrical breakdown in nanogaps, particularly in the context of nanoscale devices and applications.This review article discusses the mechanisms of electron emission and electrical breakdown in nanogaps, focusing on the transition between different emission mechanisms, the role of surface effects, and the impact of high electric fields and temperatures. It summarizes recent theories, experiments, and atomistic simulations related to these phenomena. The process of electrical breakdown in nanogaps involves nano-protrusion growth, electron emission, thermal runaway, and plasma formation. The review emphasizes the importance of understanding these processes for the design and operation of nanoscale electronic devices. Electron emission in nanogaps can occur through several mechanisms, including direct tunneling, field emission, and space-charge limited emission. The Fowler-Nordheim (FN) equation describes field emission at low current densities, while the Child-Langmuir (CL) law describes space-charge limited emission at high current densities. Recent studies have shown that quantum effects become significant at nanoscales, and the CL law can be modified to account for these effects. The review also discusses the influence of electrode geometry, surface roughness, and material properties on electron emission and breakdown. Experiments on electron emission and breakdown in nanogaps have been conducted using various techniques, including fixed and adjustable nanogap configurations. These experiments have revealed the effects of space-charge, electrode deformation, and other factors on the breakdown process. The review highlights the importance of understanding these factors for the development of reliable nanoscale electronic devices. The review also discusses the role of atomistic simulations in understanding the behavior of nanogaps. These simulations have provided insights into the growth of nano-protrusions, the thermal runaway of nano-protrusions, and the formation of plasma in nanogaps. The review concludes that further research is needed to fully understand the complex mechanisms of electron emission and electrical breakdown in nanogaps, particularly in the context of nanoscale devices and applications.