The study of molecular capsules, capable of encapsulating guest species, has grown exponentially since the early work of Pedersen and Lehn. Stemming from these seminal contributions, research in this area has been pushed forward dramatically in terms of both the design of capsules and demonstration of their function, particularly related to chemistry within the confined spaces of their internal cavities. In the design of capsule systems themselves, molecular engineering strategies have been developed to prepare architectures across scale lengths, that are robust, structurally sophisticated, and that incorporate functional units to change physical and chemical properties. In terms of function, the effects of encapsulation on the properties of guest species have been shown to stabilise reactive species or promote reactivity in benign molecules, and to allow their selective recognition or separation from mixtures.