In his article “On certain arithmetical function” in 1916, S. Ramanujan defines the arithmetic function Tau. First, we will present his results concerning the arithmetic properties of this function, and later we will restate them in terms of the modern language of modular forms. Secondly, we will discuss open questions that arose at the end of his work . Ramanujan’s achievements inspired the work of two Fields medalist J.P. Serre and P. Deligne who were the first ones to clarify the deep nature of his results. Their work gave birth to the theory of Galois representations attached to modular forms which is now a central topic in number theory and for instance, it is the main tool in A. Wiles proof of Fermat’s Last Theorem.