Sulfondiimines are aza-analogues of sulfones and sulfoximines. In contrast to the latter two compound classes, sulfondiimines are rare in the chemical literature. Although a full understanding of the stability and reactivity of sulfondiimines is wanting, sulfondiimines have recently been recognized as novel bioisosteres for carbonyl moieties enabling expansion of the well-known portfolio of pharmaceutically relevant compounds. In this review, we briefly summarize the structure and stability of sulfondiimines and then focus on syntheses and derivatisations of these interesting compounds with sulfur-nitrogen core units. Furthermore, their use in heterocyclic chemistry and recent applications as bioactive compounds are presented.