Gamma-ray bursts (GRBs) and their afterglows are created by ultrarelativistic jets with huge amounts of energy, driving a blast wave into a tenuous circumsource medium. They are therefore seen as promising laboratories of particle acceleration and relativistic magnetohydrodynamics. The observed afterglow lightcurves and spectral energy distributions give us some insight into the micro- and macro-physics at work, but, at cosmological distances, the jets themselves can not be spatially resolved. Polarimetry offers the unique capacity to diagnose the collimation of the outflows, the structure within jet and the magnetic field geometry in the emitting blastwave. Polarimetry of a very rapidly fading source is very challenging, but new technology and observing modes have been developed to tackle this. In this talk I will review our recent successes in afterglow polarimetry, and the consequences for GRB blast wave physics.