In this Thesis we use atomic force microscopy (AFM) and scanning white light interferometry (Zygo) to investigate the spatial heterogeneity of plastic flow in crystalline materials. To this end we analyze one-dimensional surface profiles obtained from surface maps of deformed Alkali Halide single crystals. The Alkali Halides investigated were potassium chloride (KCI) and lithium fluoride (LiF), both of high purity. In the case of LiF also irradiation hardened LiF was investigated. For the deformed surfaces of the pure Alkali Halides we find self-affine behaviour with an exponent of about ζ=0.7..0.8 indicating long-range correlated patterns in the underlying strain field. No intrinsic limit to the scaling is found and the combination of AFM and Zygo allow us to assess the exponent over almost 6 orders of magnitude, from the mm down to the nm scale. We also find power-law behaviour for the probability density function of the step height p(s) and the exponent found to describe the power-law best over about 3 orders of magnitude is about _T=-1.5. For p(s) an intrinsic limit is found towards larger surface step sizes. For irradiated LiF a different behaviour is observed for low deformations where strain softening leads to the emerging of characteristic step heights rather than scale-free behaviour. Towards larger strains the scale free behaviour with an exponent of _T=-1.5 is recovered.