Cucurbit[n]urils stand out among supramolecular macrocycles with respect to their recognition propensity towards cationic and neutral guests in aqueous solution. Their ability to complex guests with ultra-high affinity can be understood in terms of multiple non-covalent interactions and the hydrophobic effect, to which the release of high-energy water from the cavity contributes considerably. This chapter focuses on the structure and physical properties of CB[n], including the peculiarities of their cavity, and their host–guest complexation with charged and neutral guests.