Pyrolysis of hydrazines and their derivatives often results in homolytic cleavage of the Nsingle bondN bonds, but molecular rearrangements and eliminations are also observed in many cases. Thermal reactions in solution, in the solid state, and in the gas phase under static, flow, and flash vacuum pyrolysis conditions are considered in this review. Arrhenius or Eyring activation parameters are reported whenever available, but the original literature should always be consulted, as these parameters are often derived under widely different experimental conditions. The compound classes investigated include open-chain and cyclic hydrazine derivatives and their relationship with azomethine imines, N-amino-heterocycles, N-isocyanoamines and N-isocyanatoamines, N-aminocarbodiimides, azines, hydrazones, semicarbazides and semicarbazones, hydrazides, aminimides, amidrazones, azimines, triazenes, tetrazadienes, pentazadienes and hexazatrienes, and triazines. Understanding of reaction mechanisms is emphasized whenever possible. Reactive intermediates involved in these pyrolysis reactions include ketenes, ketenimines and other cumulenes, free radicals and diradicals, zwitterions, carbenes, nitrenes, nitrile ylides, nitrile imines, arynes and azetes.