  Field line helicity measures the net linking of magnetic flux with a single
magnetic field line. It offers a finer topological description than the usual
global magnetic helicity integral, while still being invariant in an ideal
evolution unless there is a flux of helicity through the domain boundary. In
this chapter, we explore how to appropriately define field line helicity in
different volumes in a way that preserves a meaningful topological
interpretation. We also review the time evolution of field line helicity under
both boundary motions and magnetic reconnection.
