High-speed video observations were conducted on microscale in order to reveal detailed information on the diameter, form and trajectories of nuclei that cause sheet cavitation inception on a NACA 0015 foil. The sizes found ranged from 15 to 430 um. From the observed shapes and behaviour of the nuclei it is concluded that the water in the cavitation tunnel mainly contains solid nuclei. After filtering of the water also gas and fluid nuclei were observed to cause cavitation inception. Comparison of the bubble trajectories with calculated streamlines showed increasing deviations with the size of the gas nuclei. The largest bubbles showed strong deformations due to strong deceleration and acceleration.