Abstract: We can associate to a simplicial complex of dimension d a tuple f =(f_0,f_1,…,f_d),where f_i counts the number of faces of the simplicial complex of dimension i. The tuple f is called the f-vector of the simplicial complex. In this expository talk, I will describe the Kruskal-Katona theorem, which characterizes what vectors can be the f-vector of a simplicial complex. If there is time, I will explain some connections to commutative algebra.