The study of the interaction between a magnetic field and plasma treated as a continuous medium. Most of the universe contains not normal gas, but instead plasma. Many dynamical astronomical processes are caused by the subtle nonlinear relationship between a magnetic field and plasma. There is virtually no interaction with a magnetic field in a neutral gas, but in a plasma extremely close coupling with the magnetic field means that whatever the plasma is doing intimately affects the magnetic field and vice versa. Magnetohydrodynamics builds on the tools of both fluid dynamics and electromagnetism, but it possesses many new features that are present in neither. It does consider individual particles but, instead, treats plasma as a continuous medium. The assumption of a continuous medium is valid for length-scales much larger than the mean-free path for particle collisions, which is typically 3 cm in the solar chromosphere and 30 km in the solar corona.