The paper aims to provide a brief and comprehensible overview of the current understanding of twin nucleation and growth in structural metallic materials with a hexagonal close-packed crystal structure. It describes possible experimental methods to improve this understanding, which is required to implement twin nucleation and growth in crystal plasticity models using mechanistically meaningful criteria. These aspects are further discussed by presenting results from deformation experiments carried out on a zirconium alloy (ZIRLO (TM)) and Ti-6 wt% Al-4 wt% V (Ti-6Al-4V). It is shown that very significant texture changes after even small levels of plastic deformation can be observed in both materials, strongly suggesting the presence of extensive (10 (1) over bar2) twinning. The textures produced during high-temperature rolling and compression testing demonstrate that this twinning mode is still operative at elevated temperatures. A comparison between macroscopic texture changes during compression testing and fine-scale orientation mapping using electron backscatter diffraction revealed a significant discrepancy between the expected and observed twinned volumes in ZIRLO and Ti-6Al-4V. One explanation for this discrepancy is that, in both materials, twinning is progressing to encompass entire grains. However, the required accommodation of the transformation strain due to the shear produced by the twinned grain needs to be further investigated to confirm the possibility of such a mechanism. ; Times Cited: 1 SI