This Account is about coaxing molecules into spaces barely big enough to contain them: encapsulation complexes. In capsules, synthetic modules assemble to fold around their molecular targets, isolate them from the medium for relatively long times, place them in a hydrophobic environment, and present them with functional groups. These arrangements also exist in the interior spaces of biology, and the consequences include the familiar features of enzymes: rapid reactions, stabilization of reactive intermediates, and catalysis. But inside capsules there are phenomena unknown to biology or historical chemistry, including new structures, new stereochemical relationships, and new reaction pathways. In encapsulation complexes, as in architecture, the space that is created by a structure determines what goes on inside. There are constant interactions between the container and contained molecules: encounters are not left to chance; they are prearranged, prolonged, and intense. Unlike architecture, these reversibly formed containers emerge only when a suitable guest is present. The components exist, but they cannot assemble without anything inside. Modifications of the capsule components give rise to the results of the present Account. The focus will be on how seemingly small changes in the encapsulation complexes, exchanging a C═S for a C═O, reducing an angle here and there, or replacing a hydrogen with a methyl, can lead to unexpectedly large differences in behavior.