Molecular containers have piqued the interest of supramolecular chemists since the early beginnings of the field. Cavitands’ inner cavities were quickly exploited by Cram and Warmuth to stabilize and characterize short-lived reactive species such as cyclobutadiene or o-benzyne. Since then, more complex molecular architectures have been prepared able to store and isolate a myriad of fleeting species (i.e. organometallic compounds, cationic species, radical initiators…). In this review we cover the examples of stabilization of reactive species by inclusion in molecular containers from the first reports of covalent cointaners by Cram until the most recent containers with self-assembled structure (metal coordination cages and hydrogen bonded capsules). Finally, we will focus on the examples reported by Rebek in which elusive reaction intermediates could be detected in the inner cavities of self-folding resorcin[4]arene cavitands by the formation of covalent host-guest complexes.