We study cosmic string formation within supersymmetric grand unified theories (GUTs). We consider gauge groups having a rank between 4 and 8. We examine all possible spontaneous symmetry breaking patterns from the GUT down to the standard model gauge group. Assuming standard hybrid inflation, we select all the models which can solve the GUT monopole problem leading to baryogenesis after inflation, and are consistent with proton lifetime measurements. We conclude that, in all acceptable spontaneous symmetry breaking schemes, cosmic string formation is unavoidable. The strings which form at the end of inflation have a mass which is proportional to the inflationary scale. Sometimes a second network of strings form at a lower scale. Models based on gauge groups which have a rank greater than 6 can lead to more than one inflationary era; they all end by cosmic string formation.