In type II superconductors, like the new high Tcoxide materials, if you cool acrystal below the superconducting transition in an applied magnetic field, B>Bc1, the system minimizes its free energy by distributing the flux, in units of single flux quanta, on a lattice. This should be a perfect experiment for the loveable neutron with its ability to detect variations of magnetic field in a sample. There exist several studies of these effects in conventional superconductors but, as usual for high Tcsystems, weird and wonderful things are expected. The theoreticians predict: no flux lattice only a glassy state, a lattice at low T which will melt below Tc, entangled-disentangled configurations in the liquid state, a vortex-plasma of flux-lines (whatever that may be) and so on. Its the usual story, we need some experiments to separate fact from highly entertaining theoretical models. However, the experiments are not that easy…