The reversible behaviour of metals at low applied stresses is more complex than the generally assumed linear behaviour. This is primarily because of the reversible nature of dislocation motion leading to a strain contribution known as anelasticity. This work aims to investigate (a) quantification of dislocation structures in industrial grade stainless steels, (b) unloading behaviour, and (c) the fundamentals of reversible and mechanical behaviour occurring below the yield stress. Mechanical testing of martensitic stainless steel (Stavax ESR) was performed in two different modes: incremental plastic deformation and cyclic loading-unloading below the yield stress with a focus on the measurement of small strains and corresponding stresses, occurring in the pre-yield regime. The non-linear reversible behaviour was quantitatively analysed, as opposed to the common approximation of an empirical determination of apparent Young's modulus. The recently proposed pre-yield model has been refined and, for the first time, successfully applied to a complex microstructure such as stainless steel. The quantification of dislocation structure parameters is shown to be an efficient alternative to the conventional experimental methods of quantifying dislocation structure. Further, a unique representation and quantification of the unloading and hysteresis behaviour provides more insight into the material behaviour. Lastly, the little studied microplasticity occurring below the yield stress upon cyclic loading-unloading was determined. Importantly, the physical basis of the model will allow quantification of plastic deformations in the pre-yield region, large enough to be significant in industrial processes.