When it occurs photochemically, it results in an unstable nitrene intermediate, which then sees a migration of the R group towards the nitrogen atom.
The Weinstock modification to this reaction can be utilized with diacids, and Ninomiya, Shiori, and Yamada introduced the use of DPPA in a one-step variation of this reaction to yield urethanes. When it occurs photochemically, it results in an unstable nitrene intermediate, which then sees a migration of the R group towards the nitrogen atom. The product of this reaction cannot be isolated via the similar Hoffmann rearrangement, which occurs in basic conditions. Darapsky and Hillers proposed a modification of this reaction to produce alpha amino-acids from ethyl acetoacetate and the thermal form of this reaction usually begins with an acyl halide which is attacked by an azide group from an azide salt. For 10 points, identify this reaction in which an acyl azide decomposes into an isocyanate. 
The Weinstock modification to this reaction can be utilized with diacids, and Ninomiya, Shiori, and Yamada introduced the use of DPPA in a one-step variation of this reaction to yield urethanes.
{Curtius} Rearrangement [or {Curtius} reaction]