$\oint \vec{B} \cdot d\vec{l} = \mu_0 I_{\text{enclosed}}$: the line integral of $\vec{B}$ around any closed Amperian loop equals $\mu_0$ times the net current threading through the loop. Sign convention: use right-hand rule with the loop direction to determine positive current direction. Ampere's law is always true but only useful for calculating $\vec{B}$ when the current distribution has high symmetry (infinite wires, solenoids, toroids, cylindrical conductors).