As part of a study aimed at using helium implantation to develop high levels of porosity in thin layers of silicon, we have irradiated thin silicon specimens with 6-keV He ions in the Salford in-situ TEM / ion-accelerator facility. Specimens consisted of a 100 nm layer of polycrystalline Si (poly-Si) on a 50 nm layer of SiO₂ on monocystalline Si (c-Si) prepared as a thin (≈100nm) cross-section. The in-situ experiments, thus, consisted of transmission electron microscopy (TEM) observations of these three layers under simultaneous room-temperature He irradiation, enabling detailed comparisons to be made of ion-beam-induced effects in the poly-Si and in the c-Si. Although He bubble development was similar in the two types of silicon, significant differences were noted in the formation of interstitial defect clusters in the two materials. Although, these were observed to form in the c-Si, and in the largest grains in the poly-Si, they were completely absent in the smaller grains. In addition, the poly-Si was observed to amorphise much more readily than the c-Si (at fluences of ≈ 2–3 x 10¹⁷ ions/cm²). Finally, He irradiation resulted in shrinkage of the SiO₂ layer – with a concomitant growth of Si at its interfaces with the c-Si and poly-Si. The presentation will illustrate these effects with video-clips of the experiments and will provide an interpretation of the observations that highlights the importance of grain boundaries in absorbing interstitials and in nucleating amorphous silicon.