Gas-assisted focused electron beam (FEB) and ion beam (FIB) processing and fabrication involve using electron and ion beams to directly modify materials at the nanoscale. These beams can remove or deposit material through interactions with precursor gases, enabling nanofabrication without the need for traditional lithography. The processes are used in various applications, including photomask repair, circuit restructuring, and nanophotonics. The minimum dimensions achievable are often larger than the beam diameters, and the mechanisms behind precursor gas activation are not fully understood. The review discusses the principles, fundamentals, and challenges of gas-assisted FEB and FIB processing, including the effects of electron and ion interactions, precursor gas dynamics, and the role of different beam parameters. It also covers the application fields in research and industry, as well as the current state of understanding and future prospects for these technologies.Gas-assisted focused electron beam (FEB) and ion beam (FIB) processing and fabrication involve using electron and ion beams to directly modify materials at the nanoscale. These beams can remove or deposit material through interactions with precursor gases, enabling nanofabrication without the need for traditional lithography. The processes are used in various applications, including photomask repair, circuit restructuring, and nanophotonics. The minimum dimensions achievable are often larger than the beam diameters, and the mechanisms behind precursor gas activation are not fully understood. The review discusses the principles, fundamentals, and challenges of gas-assisted FEB and FIB processing, including the effects of electron and ion interactions, precursor gas dynamics, and the role of different beam parameters. It also covers the application fields in research and industry, as well as the current state of understanding and future prospects for these technologies.