The MRI (Balbus-Hawley 1991) occurs when a weak magnetic field threads a differentially rotating disk (dΩ/dr < 0). A radially displaced fluid element connected by a field line to a neighbour loses angular momentum and falls inward — runaway instability. Growth rate γ² = −dΩ²/dr·r − (k_z v_A)², where v_A = B/√(4πρ) is the Alfvén speed. MRI drives turbulence and outward angular momentum transport, enabling accretion.