A spherical t-design on the d-dimensional sphere Sd⊂Rd+1 is a finite set of points X⊂Sd such that for any polynomial P of degree at most t, the average value of P over X is equal to the average value of P over the entire sphere Sd. For each t∈N, let C(d,t) be the minimal number of points in a spherical t-design. Establish upper and lower bounds on C(d,t) that are as strong as possible.