  Longitudinal polarisation of the lepton beam is a key ingredient to the
success of the world's unique e\pmp ring collider HERA. This article aims at
providing a brief introduction to the physics motivation for deep-inelastic
scattering of polarised electrons or positrons off protons, the basic
mechanisms to establish lepton polarisation in the high-energy storage ring and
to describe briefly the three different polarimeters, which measured both the
transverse and the longitudinal polarisation.
