Although the intermittent and heterogeneous nature of plastic flow has been known for several decades, only recently have observations performed on the surface of deformed samples by atomic force microscopy (AFM) or scanning white-light interferometry, or in the bulk by X-ray topography, revealed the scale invariant character of dislocation and slip patterns emerging from collective dislocation interactions. This scale invariance implies that the spatial fluctuations of dislocation density and/or slip never vanish as one coarsens the observation scale. An immediate consequence is that a priori obvious concepts such as “slip bands” or dislocation density can be ill-defined. These detailed characterizations of the plastic flow heterogeneity also challenge the modelling of plasticity.