AbstractÐThe purpose of this work is to thoroughly understand tension±compression asymmetry in precipitated NiTi using unique experimental results and micro-mechanical modeling. For the ®rst time, tensile and compressive stress±strain behaviors were established on aged single crystals ([100], [110], and [111] orientations) and polycrystalline NiTi. The single crystal and polycrystalline Ti±50.8 at.% Ni materials were given both peak aged and over aged heat treatments. The drawn polycrystalline NiTi has a strong texture of the h111i{110} type, thus it deformed in a manner consistent with the [111] single crystals. In contrast to the phenomenological theory of martensitic transformations (analogous to Schmid's law), the critical resolved shear stress required to trigger the transformation, t crss , in the peak-aged single crystals was dependent on both the stress direction and crystallographic orientation. Using micro-mechanical modeling, the deviation from Schmid's law was attributed to the unique orientation relationship that exists between the Ti 3 Ni 4 precipitates (their coherent stress ®elds) and the 24 martensite correspondence variant pairs. The over-aged single crystals generally obeyed Schmid's law within experimental error, consistent with the proposed micro-mechanical model. Qualitatively, the tension±compression asymmetry and orientation dependence of the recoverable strain level, e 0 , was consistent with the phenomenological theory for martensitic transformations. However, the peak-and over-aged single crystals generally both demonstrated smaller e 0 magnitudes than predicted. The dierences for both crystals were attributed to the inhibition of martensite detwinning coupled with several unique microstructural eects.