Relativistic jets are a powerful, spectacular, yet poorly-understood phenomenon associated with accreting black holes and neutron stars. The question of the internal structure - matter content and magnetic field - of relativistic jets is of prime importance for our understanding of the underlying physics. The study of circular polarisation is both challenging and potentially very rewarding, and when applied to relativistic jets has the potential to probe the structure of the jets in unique ways. This book compiles the contributions, both theoretical and observational, from a three-day workshop held in Amsterdam in July 2002, concerning the study of circular polarisation from relativistic jet sources.