<p>Magnetic tunnel junctions (MTJs) underpin modern spintronics, enabling the manipulation of electron spin for information storage, sensing and logic applications beyond conventional charge-based electronics. Among them, MgO-based MTJs have become the benchmark for high-performance spintronic devices because their crystalline barriers support coherent spin-dependent tunnelling, producing large tunnelling magnetoresistance at room temperature. This Primer reviews the materials, fabrication methods and characterization techniques that govern the performance of MgO-based MTJs, emphasizing how atomic-scale structural control enables efficient spin transport. We discuss thin-film deposition techniques such as magnetron sputtering and molecular beam epitaxy, MgO barrier formation and post-deposition annealing and lithographic patterning of multilayer stacks. Structural, magnetic and transport characterization methods, including transmission electron microscopy, X-ray diffraction, magnetometry and electrical measurements, are discussed to elucidate the correlations between interfacial structure, magnetic anisotropy and tunnelling magnetoresistance. Representative results on crystallinity, magnetic properties and transport behaviour are presented alongside switching mechanisms such as spin-transfer torque, spin–orbit torque and voltage-controlled magnetic anisotropy. Emerging directions, including antiferromagnetic tunnel junctions and van der Waals MTJs, are highlighted for their potential to overcome the limitations of conventional materials. We conclude with perspectives on reproducibility, fabrication challenges and future opportunities for ultrafast, non-volatile and energy-efficient MTJ-based technologies.</p>