Provided is a capsule having a shell of material that is a supramolecular cross-linked network. The network is formed from a host-guest complexation of cucurbituril (the host) and one or more building blocks comprising suitable guest functionality. The complex non-covalently crosslinks the building block and/or non-covalently links the building block to another building block thereby forming the supramolecular cross-linked network. The capsules are obtained or obtainable by the complexation of a composition comprising cucurbituril and one or more building blocks having suitable cucurbituril guest functionality thereby to form a supramolecular cross-linked network.