Twinning is an important deformation mechanism in hcp metals like Zirconium. It is now accepted that, unless twinning is accounted for, the typical deformation textures in these materials cannot be predicted. Current crystal plasticity models use a critical resolved shear stress criterion for twinning. However, twinning is a two-stage process involving nucleation and growth and in hcp metals nucleation and growth criteria can be very different [1] and that they might be influenced by concurrent or prior slip activity. The aim of the proposed experiments is to determine the effect of prior slip on the nucleation and growth of the most common twin in hcp alloys. This helps to develop new crystal plasticity models incorporate twinning to predict mechanical properties and texture evolutions. The work proposed is part of the ¿programme proposal¿ studying deformation mechanisms in Zr alloys.