Synthetic receptors that surround their target molecules - self assembled capsules and deep cavitands - have emerged as the most realistic models of enzymes active sites. They were introduced to study the behaviour of molecules isolated in small spaces and it has become increasingly clear that the behavior of molecules in dilute aqueous solution does not reflect their behavior in confimed spaces. The synthetic receptors fold around their target guests, isolate them from the bulk solvent, provide a hydrophobic environment and present the guests with each other in a limited space. These features combine to show high binding selectivity, large rate from the ground up; they are designed, synthesized then tested. In recent years, we have found a short-cut to total synthesis; some capsules readily insert spacer elements in the presence of suitable guests that fill the enlarged spaces. This expands the repertoire of containers and the present review describes their structures, the nature of the spaces inside, the exchange dynamics, and the rules that govern their formation.