Reactivity in nanoscale vessels is based on host–guest chemistry, the “chemistry of the inner phase.” This term was coined by Cram to describe the mechanical entrapment of molecules within other molecules, and the new reactivity patterns that could be observed on this encapsulation. He quickly realized that encapsulated molecules showed behavior that was different from what was known about them in solution and concluded that molecules (“guests”) that were trapped inside another molecule (“host”) were in essence in a different state of matter. The field of host–guest chemistry has expanded drastically since his initial findings, and new reactions within the confines of a host are discovered at an ever‐increasing rate. We will try to put relevant findings into order, in an attempt to highlight common themes and to illustrate general trends.