A review is made of the physical mechanisms of sheet cavitation inception. Next to the local pressure, also the characteristics of fluid, solid surface and flow determine the inception process. Five inception mechanisms are described with key parameters such as free-stream nuclei, roughness elements, surface-bound nuclei and a laminar separation bubble. The resulting forms of attached cavitation are designated as spot, finger or patch cavities. However, a detailed description of the nucleation process could not be made due to limitations to the spatial and temporal resolution of the available high-speed observations. Therefore, several hypotheses on the working mechanisms of the nucleation process still exist. In particular, further research is required on the contributions to the nucleation process of turbulent pressure fluctuations, mass transfer, gas expansion, evaporation and diffusion.