Evolved galaxies can be regarded as systems consisting of a vast number of stars, whose motions are governed by the gravitational potential generated by their own gravity. The theory that addresses such a system is stellar dynamics. The structure of galaxies is fundamentally explained by stellar dynamics. We first describe the basic structure of galaxies, focusing in particular on the triaxial shape of spheroidal galaxies. The two-body relaxation time for galaxies is found to be significantly longer than the age of the Universe, indicating that galaxies are collisionless stellar systems. The governing equation for a collisionless N-body self-gravitating system is the Jeans equation. We derive the Jeans equation and apply it to the spheroidal system, demonstrating that spheroidal galaxies are not supported by rotation but by velocity dispersion. An intriguing question arises here: why do all spheroidal galaxies exhibit such similar appearances, even though they are collisionless systems and cannot achieve equilibrium? The concept of violent relaxation was introduced to address this issue. However, we show that the related statistical mechanics theory is not yet well established and may not provide a strong explanation for the ubiquitous appearance of spheroidal galaxies. Finally, we discuss the dynamical properties of galaxies and present evidence for the presence of dark matter on galactic scales.