Abstractα‐Lactams have often been postulated as intermediates in numerous processes. The isolation of the first α‐lactam, N‐tert‐butyl‐3‐phenylaziridinone, was accomplished in 1962. Since then, further N‐ and C‐3‐substituted α‐lactams have been synthesized. Most of these very reactive compounds undergo easy thermal decomposition; remarkable is the relative stability of 1,3‐di‐tert‐butylaziridinone, which does not start to decompose below 140°C. The α‐lactam ring is opened by nucleophilic reagents. — The correlation of stability and substituents is discussed in the last section. An acyclic, planar or almost planar, charge‐delocalized intermediate is sugge