Planck is a European Space Agency (ESA) satellite, launched in May 2009, which is mapping the Cosmic Microwave Background anisotropies in intensity and polarisation with un-precedented detail and sensitivity. It will also provide full-sky maps of astrophysical foregrounds. An accurate knowledge of the telescope beam patterns is an essential element for a correct analysis of the acquired astrophysical data. We present the optical design of the High Frequency Instrument (HFI) together with the optical performances measured during the ground calibration campaigns. We report on the evolution of the knowledge of the pre-launch HFI beam patterns when coupled to the telescope, and on their significance for the HFI data analysis procedure.