Over the last 15 years or so, it has been shown that low-valent, electron-rich uranium(III) complexes exhibit a wide variety of reactivity towards small molecules. As a result, the field of uranium-mediated small-molecule activation chemistry has undergone significant development in recent years. The classical organometallic reactivity patterns of oxidative addition and reductive elimination that dominate the chemistry of transition-metal complexes are much less common for uranium. Owing to the invocation of the 5f orbitals for bonding and the highly polarising nature of the actinide centre, the prevalent reactivity observed for non-aqueous uranium compounds is that of migratory insertion, σ-bond metathesis and redox activity, and this can account for the often unexpected chemistry encountered with these species. This microreview focuses on the activation chemistry of trivalent uranium complexes towards the important small molecules dinitrogen (N2), nitric oxide (NO), azide (N3–), carbon monoxide (CO) and carbon dioxide (CO2).