Charged particles can be confined by electromagnetic fields to a finite volume in space for extended periods of time. The extremely long observation time along with the nearly perturbation-free environment under ultra-high vacuum conditions opens new regions of spectroscopy, which are not accessible by other methods. In microwave spectroscopy resonance lines as narrow as Δv/v = 3 × 10−12have been obtained, which leads directly to the possible development of a new type of frequency standard. Lifetimes of metastable levels in the region of many seconds can be observed, as well as rare collision processes between ions and molecules, which are important for the formation of larger molecules in interstellar space. Finally, phenomena like crystallisation in a non-neutral plasma may be investigated at sub-Kelvin temperafures. accessible by the new technique of laser cooling of ions.