In many cases theory has to resort to approximations to treat few-body systems. These approximations have to be tested by detailed experimental data. Atomic collisions or precision laser spectroscopy are particularly suitable for such tests. The fundamental force underlying atomic systems, the electromagnetic force, is essentially understood. Therefore, any discrepancy found between experiment and theory can be directly related to the theoretical description of few-body effects, or to the existence of new fundamental forces (beyond-Standard-Model forces). In nuclear systems, in contrast, the underlying force is much less understood. Furthermore, in atomic collisions the number of particles can be kept small enough so that complete kinematic information about every single particle in the system can be obtained experimentally (see article on kinematically complete experiment). In systems with large particle numbers, in contrast, usually only statistically averaged or collective quantities about the system can be measured.