Physics of $\Upsilon$ resonances: ten years later

This paper reviews the present status of the physics of bottomonium: a bound system consisting of a heavy $b$ quark and the antiquark $\bar b$. The basic experimental data on the levels of bottomonium are presented. Theoretical methods for describing the properties of these levels are discussed. Questions pertaining to the spectroscopy of bottomonium, including the fine and hyperfine splittings, radiative transitions between levels, and annihilation decays of the $b\bar b$ system, are discussed. Effects which are not describable by quantum-chromodynamics perturbation theory are taken into account. Transitions between bottomonium levels involving the emission of light muons are discussed. The possibilities of a search for hypothetical new particles and effects in the decays of $\Upsilon$ resonances are also discussed.