Catalysed X–H insertion reactions into diazo compounds (where X is any heteroatom) are a powerful yet underutilized class of transformations. The following review will explore the historical development of X–H insertion and give an up-to-date account of the metal catalysts most often employed, including an assessment of their strengths and weaknesses. Despite decades of development, recent work on enantioselective variants, as well as applying catalytic X–H insertion towards problems in chemical biology indicate that this field has ample room for innovation.