Shape Memory Alloys (SMAs) are known for their smart properties connected to a thermoelastic martensitic transformation, which gives rise to the pseudoelasticity and the shape memory effect. High strain values can be built up and recovered during thermal cycles under a constant stress state. During the transformation, an electrical resistance change is observed as the sum of a geometrical contribution and a term due to the modification of the electronic structure of the transformed fraction. Great interest is focused on SMAs thanks to the dependence of the electrical resistance on the strain during the transformation, which embodies a great appeal for sensing/actuating devices. However, a linear relationship in the ER vs. curve is found only when a single transformation is present and both the resistivity change and the geometrical contribution are linearly dependent on the transformed fraction. In this case a small hysteresis in the ER vs. E curve can be achieved. The same does not hold when more than one transformation are present; in this case a large hysteresis appears and the sensing/actuating function is lost. Some specific transformations in two of the most interesting SMAs alloys, i.e. NiTi and NiTiCu, are here examined to identify the basic features required for sensing/actuating purposes.