[Truncated abstract] The study of NiTi shape memory alloys, although comprehensive and diverse, still encounters numerous uncertainties and misunderstandings that often jeopardise the effective use of these alloys in various applications. One such key area is the understanding of the micromechanics and thermodynamics of the deformation mechanisms, such that their deformation behaviour can be accurately predicted and modelled. Furthermore, most research involves polycrystalline NiTi of varying compositions and processing history, both of which complicate and damage the internal structure of the matrix even before deformation. This work aims to study the micromechanisms of deformation of near-equiatomic NiTi alloys, both in polycrystalline and single crystal forms, with particular attention given to the commonly observed phenomena of Luders-like deformation behaviour and deformation induced martensite stabilisation. This work was carried out in three sections. Firstly, the tensile deformation of polycrystalline NiTi samples via martensite reorientation and stress-induced martensitic transformations was carried out. The samples were deformed to various stages of deformation and then thermally cycled to study the thermomechanical response to deformation as a means to explore the various mechanisms of deformation. Next, the deformation and post-deformation transformation behaviour of NiTi single crystals were studied to verify the effect of grain boundaries and other hypotheses raised regarding the deformation mechanisms. The single crystal samples were deformed along three low-index axial orientations. Finally, microscopic analysis was carried out on as-annealed and the deformed polycrystal and single crystal samples by means of transmission electron microscopy. The microstructural analyses accompanied the thermodynamic study and provided evidences to support various hypotheses