We use state of the art ab initio calculations to obtain the diffusion properties of He in tungsten. The calculated migration energy of He is very low, around 0.06 eV. This value is much lower than the experimental field-ion microscopy results which lead to a migration energy of the order of 0.24-0.32 eV. The reason for this discrepancy is the high propensity for He to form He-He clusters characterized by a very large binding energy of the order of 1 eV. Such a large binding energy indicates that He atoms can be trapped by other He atoms and can explain the formation of He blisters close to the surface of He implanted tungsten.