The nature of compact parsec-scale knots in AGN: clues from their polarisation properties

After reviewing early observations that helped fuel enthusiasm for interpreting compact VLBI knots as relativistic shocks, the paper considers more recent evidence that not all compact features correspond to shocks. In at least some cases, it appears that the magnetic field structures we are seeing in the VLBI jets predominantly reflect the instrinsic fields of the jets themselves. This represents a major shift in our understanding of the polarisation structure of VLBI jets, and has important implications for the interpretation of VLBI images in the framework of jet models. (C) 2003 Elsevier B.V. All rights reserved.